On that note, we need to be able to make coins out of anything. I want to have iron coinage, and bronze and stuff like that. Copper, silver, gold only is fail. Make them out of any material, and you can set the actual monetary value through a screen like the z menu or whatever. Adding to eternal suggestions.
You can make coins out of anything already, no modding needed. They just won't be used for money(which is, imo, currently an advantage if you like coins but want to both use the economy and avoid epic coin scattering).
My last fort minted one stack of electrum coins a year and sealed them in a section of the vaults. An older fort had a noble who liked coins, so I minted them in dwarfy iron when the mandates came up and stashed them in the vault. You could also mint commemorative coins for significant events.
Setting their value, however, strikes me as absurd - not unless we can have asset-backed currencies. As-is they get their value from their composition(and quality/etc modifiers) - or should, anyway. What you're asking for would seem for like a fiat-based currency system, which I'd say is pretty inappropriate for the setting(well, atleast for external trade).
I mean, how realistic would it be to buy out a large caravan with a single stack of tin coins, just because your dwarves claim they're worth, say, ☼150'000?