PaulS:
A PIR sensor is useless for detecting cold metal coins. PIR sensors detect people, so you'll need to set your apparatus up to count people carrying coins. Good luck with that.
At the risk of being seen to split hairs, a PIR sensor may work if the coins are cold (relative to the ambient temperature). The problem arises when these are at exactly the ambient temperature. The rest stands, of course.
The OP should look at say IR beam break or proximity sensors if the detection using PIR sensors proves to be unreliable.