Like most pseudo-medieval fantasy worlds, the common form of money is in minted coins of precious metals. Each nation mints its’ own coins of course, and the actual amount of precious metal in the coins may vary somewhat, but thanks to the lingering standardizations of the old Lyiertian Empire, players may treat the vast majority of coinage as pretty much interchangeable without getting into the fiddly business of currency exchanges.
A single coin is roughly the size of a nickle and weighs 1/100th of a pound.
In truth their actually a bit heavier than that, but 100 coins to the pound is a nice even number.
A copper coin is worth $0.10 or less.
A silver coin is $1.
A gold coin is $20.
A platinum coin is $100.