My understanding is that gold and silver as a raw material has a set value.  I mean one big bar of either will be the going trade value on the board.  Talking millions of dollars of gold or silver.

Smaller quanities the value changes depending on what is stamped on it, by who, how many made, how long ago it was made, ect.  Why does it change the value?  Because we as humans want it to change the value, that is it. 

So I have always bought coins that are common.  Like US or Canadian and stayed away from the unusual stuff.  Yes, I pay a bit of a premium, but it will be more accepted when and if I need to use it.   Life.