Some motors are worth breaking down, some are not. anything with AL windings is not. As far as copper content, its something you have to get a feel for. As you break down more and more you will begin to understand which ones are worth the time for a few dollars and which ones are not. Experience is the key.

Other people's estimates are good but each motor is made different. there is an average range, but its not a set number. the best answer would be something like "15%-30%" depending on the model. but only you would know from the feel of it and what it looks like. Good luck!