Nice and easy:
Gold = 10 Silver
1 Silver = 100 Copper
1 Copper = the hourly wage for your average Joe.
A piece of bread will always be a copper or two around your average npc town. (People are generally 'poor' in Eria.) Rent for a small place is around 50 copper a month. Other than that, the economy is completely reliant on characters, both npc and rpc, as they are the ones who put a price or something.
Money often attracts unwanted attention.