They can hit anywhere from zinc-penny on up the scale.
One manufacturer (Fisher), printed the results of a test where 259 gold rings, all different sizes and alloys were scanned and the results were as follows:
FOIL: 92 (36%)
Round Tab: 51 (20%)
Nickel: 26 (10%)
Square Tab: 75 (29%)
Clad Coin: 15 (6%)
I know that this adds up to 101%... for my part, the nicest (most valuable) rings that I've dug were lady's rings with nice stones. They hit like foil on my machines (just under nickel on the 250).
If you dig for foil, as I often do, you will avoid the bulk of the trash and the trash that fools you is usually shallow and easy to recover. Just my $.02.
Skillet