Marv has the right idea... let Webster tell it like it is.

http://www.m-w.com/dictionary.htm

Main Entry: coin
Pronunciation: 'koin
Function: noun
Etymology: Middle English, from Middle French, wedge, corner, from Latin cuneus wedge
Date: 14th century
1 archaic a : CORNER, CORNERSTONE, QUOIN b : WEDGE
2 a : a usually flat piece of metal issued by governmental authority as money b : metal money c : something resembling a coin especially in shape
3 : something used as if it were money (as in verbal or intellectual exchange) <perhaps wisecracks... are respectable literary coin in the U.S. -- Times Literary Supplement> <would repay him with the full coin of his mind -- Ian Fleming>
4 : something having two different and usually opposing sides -- usually used in the phrase the other side of the coin
5 : MONEY <I'm in it for the coin -- Sinclair Lewis>

I do believe 2c is an accepted usage of the word in this case. But then we do have a lot of 4 on this board, eh? <VBG>