bigall42, d-dancer has good and logical answer. I would add the following: You don't see any comparative charts like what you're looking for (for either ferrous or non-ferrous), because of the following reasons:
a) Because it is size relative. There's no one single "depth" on a certain type of metal. You have to have a size involved. Ie.: a coin-sized item ? A domino sized item? A soda can sized item? A volkswagon ? etc...
b) Even once you set a "standard" test (think you can test a variety of machines on the same target, and make a spread-sheet of comparative results), it's still speculative and arbitrary. Because to make it equal and fair, the tester has to have a relative sensitivity setting on each machine. And no two manufacturers have calibrated sens. level settings. Ie.: mid-way on the dial on one machine, may equate to 3/4 setting on another machine, and so forth.
c) And go figure: if you, for example, (to get a equal sens. testing benchmark basis) set each one on "maximum" sens, then .... seriously now .... there is no soil on earth that would be akin to testing through thin air. There's machines that can be made to do AMAZING air tests, but you can never replicate those results in the ground.
d) the point at which an object ceases to be heard in an air test (and ground tests), is also highly dubious . One person may insist "I still hear it" (because he hears a random flutter). While the other says "no, it's faded to the point of non-useful info". And subtleties like how fast you wave the target (or how slow) can make subtle differences on depth. And slight tilts in the object as you wave, etc.... And do you count one-way signals as "still being there" ?
Thus is the reasons you will never see a "Consumer reports" side-by-side comparison charts, like in the way you'd compare stereos, or television sets, or cars, etc...
