on what kind of batteries you are using now. I am assuming that you are talking about 2700 mAh alkaline batteries.
The Minelab NiMH (Nickel Metal Hydride) pack is composed of 1600 mAh AA cells. Energizer AAs are 2500 mAh and Duracell AAs are 2850 mAh cells. The 2700 mAh batteries that you spoke of will outlast the NiMH rechargeable pack and the Energizers but not the Duracells. If I recall correctly, the digital camera batteries are more expensive (sales gimmick) than the Duracells and won't last quite as long.
Having said all of that, There are 2700 mAh NiMH rechargeable AA cells available. These give you near the amount of runtime of the Duracells with the perk of being relatively fast to recharge, This will save you money in the long run. Go to www.batteriesplus.com and do a search for NiMH and you will find them (they may have even higher capacity cells now). We have a local store and the guys were always helpful with building custom packs for my RC stuff. Also, you can often find "off brands" on ebay for about a dollar a cell. I have never had a bad experience with a brand-x battery but YMMV. Chargers on ebay are about 20 bucks or so. So, for about 50 bucks give or take you can have two sets of NiMH batteries and a charger inaddition to the one that came with your detector. Each set of them will outperform the standard Minelab pack by 69% as will the 2700 mAh alkaline digital camera batteries. The camera batteries are just not rechargeable.
Here is the low down if you are interested (and do not know this stuff already):
mAh (milli-ampere hour) is a capacity rating that measures how much current a battery will discharge over a specified period of time (typically a one hour period). Higher mAh ratings do not necessarily reflect how fast current can be drawn, rather, how long a current can be drawn. For example a 2700 mAh battery (aftermarket NiMh and digital camera batteries) will sustain a 2700 milli-Amp (2.7 ampere) draw for approximately one hour before dropping to a voltage level that is considered discharged. A 1600 mAh battery (the stock Minelab battery pack) will sustain a 1600 milli-Amp (1.6 ampere) draw for approximately one hour. Overall capacity will be influenced by other factors including temperature, depth of discharge and speed of discharge, but all things being equal, this is how it works.
Hope this helps.
HH