I've whipped together the start of a basic handout for players. The numbers are kinda random right now, so I'll probably change them. Just wanted to get my thoughts down in writing. I won't be running a game for a while, so there's no rush.
The difficulty is scaling the values to accomodate 1st level and 20th level characters and the expected wealth they might have or discover. That's where the plastic bullion bars and diamonds and stuff come in with values in the thousands of gp.
Also, plastic coins seem to come in two sizes (small and large) and three colours: copper, gold, silver. So I'm kinda cheating and saying that the copper plastic coins are 1gp, and the gold coloured ones are higher denomination coins. The silver coloured ones are platinum.