Expect $.06 to $.11 per lb on the cases if steel. Some are aluminum of course. Quick test would be to hit them with a magnet - if it sticks/pulls the case is steel. If not it is more than likely aluminum.
And don't forget the small plate on the rear with the openings for mice and keyboards that are purple/green colored. Check them with a magnet because about 40-50% are stainless. Also watch out for the brass motherboard stand-off's in the older computers. All that small stuff add's up.