First of all, it's very difficult to answer that question, if not impossible. The answer really is "it depends"
The following all play a significant roll, and in combination will always give varying results:
-size of a target-> the larger the target, the deeper a target can be recognized.
-conductivity-> aluminum vs. copper vs. gold. vs. iron vs. silver. Some rings, with size & mass being equal can be detected deeper then others.
-size of coil-> general rule says "larger coil = deeper depth however, add a lot of conductive particles, metal junk and high mineralization to the matrix, and the large coil becomes redundant.
-type of coil-> DD vs.concentric vs. mono. Each coil has a definite advantage over the other under different conditions
-frequency of signal from coil-> higher frequency usually (but not always) is more sensitive to tinier objects.
-VLF vs. Pulse Induction-> Pulse induction can easily pick up a dropped mini or gold ring with a medium sized coil 15-20" whereas a VLF might get about half that all other things being equal.
-Humidity, moisture in soil, orientation of target to the coil
--ability of the to perceive the receiving frequency from a coil and distinguish it's conductivity, depth, size etc.
-operator - properly setting up the electonic components based on knowledge and experiences
There are many more variables in involved affecting depth.....the above are just some of the more obvious ones. So really, YOU need to figure out what you are looking for ie. relics vs. jewelery vs. common coins vs. deep copper/silver coins. You also need to know the soil conditions and other factors in YOUR area where you are going to hunt. Lots of electronic interference from above ground or below will make some of the most expensive machines useless, unless you turn the sensitivity way down, which often times reduces your depth considerably. Same goes with extremely high mineralization like in Australia or red clay oxidized clays in North America areas. VLF may really struggle under these conditions, yet a PI (pulse induction) will punch down deep with no trouble.
So.....how deep is the Pro? It is as deep in some instances and deeper in other instances and perhaps not as good as machines costing sometimes more then twice the cost of the . Throw on the 9 x 12 concentric coil in good ground and you have a very deep detector with great ID capabilities. Use the DD 8.5" x 12" coil in heavier mineralized soil, and it outperforms even the larger coil I just mentioned. Put on the 5" x 8" coil and you can easily pick out the good targets beside and often time underneath junk targets, while still getting great depth.
You want a machine for caches buried in feet, not inches - Garrett has it - http://www.garrett.com/hobbysite/1611800_treasurehound_eagleeye_searchcoil.aspx
You want extreme depth for water or land, check out the Garrett Infinium - http://www.garrett.com/hobbysite/hbby_infinium_video.aspx or Infinium Website http://www.findmall.com/list.php?62
You want a great all around machine for good depth + Salt or freshwater hunting (waterproof) Check out the AT Gold or : Gold: http://www.findmall.com/read.php?79,1615363,1615363#msg-1615363
You want to hunt common coins with sizing ability, check out the GTI Series- http://www.garrett.com/hobbysite/hbby_gti_1500_video.aspx
Why should depth not be the most important factor for purchasing a metal detector? Because depth is only relative to certain conditions. Most targets are in the top 6 inches anyway, so why do you need to go down to 15-20"? In places like salt water or areas with a lot of deep junk rusty targets, the large coil on your $1,500.00 machine is practically useless, and when set up to it's maximum potential in these areas, will only give you say 4-5 inches. If somebody is trying to sell you an expensive detector, that they say is the deepest machine, you had better turn around and run fast....as that equates with someone trying to sell you a a parcel of land in a swamp. There is no 'DEEPEST" machine....it simply doesn't exist. But there are BEST configurations of a metal detector type for any given matrix of soil.
As this is a Garrett forum, and I don't promote detector comparisons here, I will speak specifically on Garrett Detectors. They are one of the oldest and ORIGINAL designers, manufacturers of metal detectors in USA. They also have one of if not THE best customer service backups. Talk with the Garrett folks about a problem, and you are probably speaking with someone who also spends some of their spare time metal detecting. In fact, Garrett encourages their staff to sign out machines and use them ad lib on their days off. Garrett is up on the latest technology. They are constantly doing R & D to keep up with the newest and latest technologies. You won't find any cheap toggle switches and cheap knobs on the latest detectors. Toggles can easily break off, and knobs for tuning or volume wear out or change setting with the slightest accidental touch. Knobs are old technology and belong on static devices such as old TVs, radios etc. Garrett builds a variety of detectors. Want extreme depth? They have it....want a good all around machine with good depth and all around performance and water proof costing half of what other competitors have, Garrett has it. Want a low cost entry machine, with bells & whistles costing much less then the competitors...Garret has it. Do you need further support on detecting with any Garretts, they have it.....Garrett has many "HOW TO" videos on line for each model they sell.. Need some reading material? Garrett has the largest supply of home grown (written) books. Below is a list of all the detectors manufactured by Garrett.
1964
BFOs (zero drift)
The Hunter
The Provider
The Hustler
The Achiever
The Sidewinder
The Playmate
The Finder
The Seeker
The Shark (underwater detector)
1965
BFOs (zero drift)
The Hunter
The Provider
The Hustler
The Achiever
The Sidewinder
The Playmate
The Finder
The Seeker
The Shark (underwater detector)
1966
BFOs (zero drift)
The Hunter
The Provider
The Hustler
The Achiever
The Sidewinder
The Playmate
The Finder
The Seeker
The Shark (underwater detector)
1967
BFOs (zero drift)
The Hunter
The Provider
The Hustler
The Achiever
The Sidewinder
The Playmate
The Finder
The Seeker
The Shark (underwater detector)
1968
BFOs (zero drift)
The Hunter
The Provider
The Hustler
The Achiever
The Sidewinder
The Playmate
The Finder
The Seeker
The Shark (underwater detector)
1969
BFOs (zero drift)
The Hunter
The Provider
The Hustler
The Achiever
The Sidewinder
The Playmate
The Finder
The Seeker
The Shark (underwater detector)
1970
BFOs (zero drift)
The Hunter
The Provider
The Hustler
The Achiever
The Sidewinder
The Playmate
The Finder
The Seeker
The Shark (underwater detector)
1971
BFOs (zero drift)
The Hunter
The Provider
The Hustler
The Achiever
The Sidewinder
The Playmate
The Finder
The Seeker
The Shark (underwater detector)
The Hombre (BFO) 1971
1972
BFOs (zero drift)
The Hunter
The Provider
The Hustler
The Achiever
The Sidewinder
The Playmate
The Finder
The Seeker
The Shark (underwater detector)
*BFO models: Controls on side, no push button or toggle switch at the end of handle:
*Master Hunter BFO
*Coin Hunter BFO
*Cache Hunter BFO
*Treasure Hunter BFO
*Nugget Hunter BFO
*Mini Hunter Ghostowner BFO
*Garrett Mini-Hunter BFO
1973
*BFO models: Controls on side, no push button or toggle switch at the end of handle:
*Master Hunter BFO
*Coin Hunter BFO
*Cache Hunter BFO
*Treasure Hunter BFO
*Nugget Hunter BFO
Master Hunter TR (controls on side, made with push button)
Money Hunter TR
Mini-Hunter TR
Super Mini-Hunter TR
Super Mini-Hunter BFO
1974
*BFO models: Controls on side, no push button or toggle switch at the end of handle:
*Master Hunter BFO
*Coin Hunter BFO
*Cache Hunter BFO
*Treasure Hunter BFO
*Nugget Hunter BFO
Master Hunter TR (controls on side, made with push button)
Money Hunter TR
Super Mini-Hunter TR
Super Mini-Hunter BFO
Mini-Hunter Ghostowner TR
1975
*BFO models: Controls on side, no push button or toggle switch at the end of handle:
*Master Hunter BFO
*Coin Hunter BFO
*Cache Hunter BFO
*Treasure Hunter BFO
Master Hunter TR (controls on side, made with push button)
Money Hunter TR
Relic Hunter BFO (no push button or toggle switch)
Money Hunter BFO (no push button or toggle switch)
Coin Hunter TR (TR discriminator on side with push button)
Cache Hunter TR (TR discriminator on side w/push button)
Relic Hunter TR (TR discriminator on side w/push button)
Ghostown Hunter TR (no push button)
1976
*BFO models: Controls on side, no push button or toggle switch at the end of handle:
*Master Hunter BFO
The Coin Hunter VLF/TR (controls on side, push button)
1977
*BFO models: Controls on side, no push button or toggle switch at the end of handle:
*Master Hunter BFO
*Coin Hunter BFO
Money Hunter TR: (w/push button)
Money Hunter BFO (no push button or toggle switch)
Coin Hunter TR (TR discriminator on side with push button)
The Coin Hunter VLF/TR (controls on side, push button)
The Coin Hunter VLF/TR featherweight series (controls on top, push button, groundhog on coil)
Master Hunter VLF/TR (controls on side, push button, deepseeker on coil)
Master Hunter VLF/TR featherweight series (controls on top, push button, groundhog on coil)
Fortune Hunter TR (controls on side, push button)
1978
*BFO models: Controls on side, no push button or toggle switch at the end of handle:
*Master Hunter BFO
The Coin Hunter VLF/TR (controls on side, push button)
The Coin Hunter VLF/TR featherweight series (controls on top, push button, groundhog on coil)
Master Hunter VLF/TR (controls on side, push button, deepseeker on coil)
Master Hunter VLF/TR featherweight series (controls on top, push button, groundhog on coil)
Fortune Hunter TR (controls on top, push button)
Money Hunter VLF/TR (controls on top, push button, groundhog on coil)
1979
Master Hunter ADS (VLF/TR deepseeker control on side, toggle switch)
Coin Hunter ADS (VLF/TR deepseeker-contols on side, toggle switch)
Marksman P.B. (VLF/TR deepseeker-controls on side, push button)
Master Hunter ADS (VLF/TR Groundhog controls on top, toggle switch)
Coin Hunter ADS (VLF/TR Groundhog-controls on top, toggle switch)
Fortune Hunter P.B.(VLF/TR Groundhog-controls on top, push button)
Money Hunter P.B. (VLF/TR Groundhog-controls on top, push button)
International TR S2 (controls on top, push button)
Treasure Hunter BFO (controls on side, no push button)
1980
Master Hunter ADS (VLF/TR deepseeker control on side, toggle switch)
Coin Hunter ADS (VLF/TR deepseeker-contols on side, toggle switch)
Marksman P.B. (VLF/TR deepseeker-controls on side, push button)
Master Hunter ADS (VLF/TR Groundhog controls on top, toggle switch)
Coin Hunter ADS (VLF/TR Groundhog-controls on top, toggle switch)
Fortune Hunter P.B.(VLF/TR Groundhog-controls on top, push button)
Money Hunter P.B. (VLF/TR Groundhog-controls on top, push button)
International TR S2 (controls on top, push button)
Master Hunter BFO (controls on side, no push button)
Garrett BFO (controls on side, no push button)
1981
Master Hunter ADS (VLF/TR deepseeker control on side, toggle switch)
Marksman P.B. (VLF/TR deepseeker-controls on side, push button)
Master Hunter ADS (VLF/TR Groundhog controls on top, toggle switch)
Fortune Hunter P.B.(VLF/TR Groundhog-controls on top, push button)
Money Hunter P.B. (VLF/TR Groundhog-controls on top, push button)
American TR S2 (controls on top, push button)
Sea Hunter
1982
Master Hunter BFO (controls on side, no push button)
Sea Hunter
Master Hunter ADS III (VLF/TR Deepseeker-controls on side, toggle switch)
Master Hunter ADS II (VLF/TR Deepseeker-controls on side, toggle switch)
Master Hunter ADS I (VLF/TR Deepseeker-controls on side, toggle switch)
Master Hunter ADS I (VLF/TR Groundhog-controls on top, toggle switch)
Gold Hunter A2B (groundhog on coil, controls on top, toggle switch)
American S4 ADS (groundhog on coil, controls on top, toggle switch)
American S3 (VLF/TR, groundhog on coil, controls on top, push button)
American TR S1 (deep scan on coil, controls on top, push button)
Pocket Scanner
1983
Sea Hunter
Master Hunter ADS III (VLF/TR Deepseeker-controls on side, toggle switch)
Master Hunter ADS II (VLF/TR Deepseeker-controls on side, toggle switch)
Master Hunter ADS I (VLF/TR Deepseeker-controls on side, toggle switch)
Master Hunter ADS I (VLF/TR Groundhog-controls on top, toggle switch)
American S4 ADS (groundhog on coil, controls on top, toggle switch)
American S3 (VLF/TR, groundhog on coil, controls on top, push button)
American TR S1 (deep scan on coil, controls on top, push button)
Pocket Scanner
1984
Sea Hunter
Master Hunter ADS 7 (controls on side, toggle switch)
Master Hunter ADS 6 (controls on side, toggle switch)
Master Hunter ADS 5 (controls on side, toggle switch)
Master Hunter ADS 4 (controls on side, toggle switch)
American AM-2 (controls on top, toggle switch)
American AM-1 (controls on top, push button)
Freedom 1 Coin Commander
Freedom 2 Coin Commander
1985
Sea Hunter
Master Hunter ADS 7 (controls on side, toggle switch)
Master Hunter ADS 6 (controls on side, toggle switch)
Master Hunter ADS 5 (controls on side, toggle switch)
Master Hunter ADS 4 (controls on side, toggle switch)
Freedom 1 Coin Commander
Freedom 2 Coin Commander
Freedom Ace Coin Commander
Gold Hunter (controls on top, toggle switch)
1986
Master Hunter ADS 7 (controls on side, toggle switch)
Master Hunter ADS 5 (controls on side, toggle switch)
American AM-2 (controls on top, toggle switch)
Freedom 1 Coin Commander
Freedom 2 Coin Commander
Freedom Ace Coin Commander
Gold Hunter (controls on top, toggle switch)
Beach Hunter AT3
Sea Hunter XL 500 Pulse
Sea Hunter XL 200 Pulse
1988
Master Hunter ADS 5 (controls on side, toggle switch)
Sea Hunter XL 500 Pulse
Grand Master Hunter
Freedom 3 CDC
Freedom 1 CDC
Beach Hunter AT4
Gold Hunter A3B
1989
Master Hunter ADS 7 (controls on side, toggle switch)
Master Hunter ADS 5 (controls on side, toggle switch)
Sea Hunter XL 500 Pulse
Freedom 3 CDC
Beach Hunter AT4
Gold Hunter A3B
Grand Master Hunter II
Freedom 3
Freedom 1
1990
Sea Hunter XL 500 Pulse
Beach Hunter AT4
Grand Master Hunter CX II
Master Hunter CX
Master Hunter 7X
Freedom 3 Plus
Freedom 2 Plus
Freedom Ace Plus
Scorpion Gold Stinger
1991
Sea Hunter XL 500 Pulse
Beach Hunter AT4
Grand Master Hunter CX II
Master Hunter CX
Master Hunter 7X
Freedom 2 Plus
Freedom Ace Plus
Scorpion Gold Stinger
Ultra GTA 1000
Ultra GTA 500
Pocket Probe
1992
Sea Hunter XL 500 Pulse
Grand Master Hunter CX II
Master Hunter CX
Freedom 2 Plus
Freedom Ace Plus
Scorpion Gold Stinger
Ultra GTA 1000
Ultra GTA 500
Master Hunter CX III
1993
Sea Hunter XL 500 Pulse
Grand Master Hunter CX II
Master Hunter CX
Freedom 2 Plus
Freedom Ace Plus
Scorpion Gold Stinger
Ultra GTA 1000
Ultra GTA 500
Master Hunter CX III
1994
Sea Hunter XL 500 Pulse
Grand Master Hunter CX II
Master Hunter CX
Ultra GTA 350
Freedom Ace Plus
Scorpion Gold Stinger
Ultra GTA 1000
Ultra GTA 500
Master Hunter CX III
1995
Sea Hunter XL 500 Pulse
Grand Master Hunter CX II
Master Hunter CX
Ultra GTA 350
Freedom Ace Plus
Scorpion Gold Stinger
Ultra GTA 1000
Ultra GTA 500
Master Hunter CX III
1996
Sea Hunter XL 500 Pulse
Grand Master Hunter CX II
Master Hunter CX
Freedom 2 Plus
Scorpion Gold Stinger
Master Hunter CX III
GTAX 1000
GTAX 500
GTAX 400
1997
Sea Hunter XL 500 Pulse
Grand Master Hunter CX II
Master Hunter CX
Freedom 2 Plus
Scorpion Gold Stinger
Master Hunter CX III
GTAX 1000
GTAX 500
GTAX 400
GTI 2000
Treasure Ace 100
Treasure Ace 200
Treasure Ace 300
1998
Sea Hunter XL 500 Pulse
Master Hunter CX
Scorpion Gold Stinger
Master Hunter CX III
GTAX 1000
GTAX 500
GTAX 400
GTI 2000
Treasure Ace 100
Treasure Ace 200
Treasure Ace 300
GTI 1500
Pocket Probe
1999
Scorpion Gold Stinger
Master Hunter CX
Master Hunter CX III
GTAX 400
Treasure Ace 100
Treasure Ace 200
Treasure Ace 300
GTI 1500
Pocket Probe
GTI 2500
GTAX 550
GTAX 1250
GTAX 750
Sea Hunter Mark II
2000
Master Hunter CX Plus
Scorpion Gold Stinger
Master Hunter CX III
GTAX 400
Treasure Ace 100
Treasure Ace 200
Treasure Ace 300
GTI 1500
Pocket Probe
GTI 2500
GTAX 550
GTAX 1250
GTAX 750
Sea Hunter Mark II
2001
Master Hunter CX Plus
Scorpion Gold Stinger
GTAX 400
Treasure Ace 100
Treasure Ace 200
Treasure Ace 300
GTI 1500
GTI 2500
GTAX 550
GTAX 1250
GTAX 750
Sea Hunter Mark II
2002
Master Hunter CX Plus
Scorpion Gold Stinger
GTAX 400
Treasure Ace 100
Treasure Ace 300
GTI 1500
GTI 2500
GTAX 550
GTAX 1250
GTAX 750
Sea Hunter Mark II
2003
Master Hunter CX Plus
Scorpion Gold Stinger
GTAX 400
Treasure Ace 100
Treasure Ace 200
Treasure Ace 300
GTI 1500
GTI 2500
GTAX 550
GTAX 1250
GTAX 750
Sea Hunter Mark II
2004
Master Hunter CX Plus
Scorpion Gold Stinger
GTAX 400
Treasure Ace 100
Treasure Ace 300
GTI 1500
GTI 2500
GTAX 550
GTAX 1250
Sea Hunter Mark II
Infinium LS
GTP 1350
2005
Master Hunter CX Plus
Scorpion Gold Stinger
GTI 1500
GTI 2500
GTAX 550
Sea Hunter Mark II
Infinium LS
GTP 1350
Ace 150
Ace 250
2006
Master Hunter CX Plus
Scorpion Gold Stinger
GTI 1500
GTI 2500
GTAX 550
Sea Hunter Mark II
Infinium LS
GTP 1350
Ace 150
Ace 250
ACE 350
AT Pro
AT Gold