So many variables its just not not worth mentioning any figures,quoting a single coin depth is a total waste of time,one is not going to get any depth on say a roman minim which is crazy small as i could on say a UK Cartwheel penny which weighs 1oz,the orientation of the coin in the ground,ground conditions and also things like nulled signals from close proximity too iron and many more can cause.
Most detector with the same size coil ie 10-12'' dia are pretty even these days on finding the same size coin targets which on the whole these coins are pretty unified in size and weight,but artifact and hoards are not all the same size so the depth of these cannot really be quoted as such,i could say that i hit a Brone Age Palstave axe at 24'' with a machine and this is mainly due to the massive size of a axe head and also it being Bronze,also a hoard of BA axes or tools could be hit with a specialised machine say a Pulse with a large Mono coil at greater depths or even a twin box setup.
Most folks who have posted have mentioned coins and these have all been within say a few inches of one another,but these are pretty reliable figures i guess,but hunting for artifacts or hoards is a different ball game,you just cannot give any figures and even if you do most folks would call you a liar and vastly over exaggerating so no point in quoting any real depth figures anyway.
Detecting is very similar in many aspects to 'Fishing'
exaggeration wise it was 'that deep or it was that big' honest
This is why i dont quote figures,i would rather be out detecting as you dont find items stuck behind a computer screen.