I am a new Dming (only been Dming and playing for 2 years; actually I have been Dming for two years and I have NEVER got to play as a PC, I think I have the PC BLUES), but i have ( at least thought about) implemented this (i havent actually implemented these rules (about economics, currency), but I have thought about them, my PC's arent to creative...). You can make it as complex as you want (IT REALLY IS UP TO THE DM, if you want to keep track then do it, dont expect the PC's to keep track of the system). Mutiple currency is a good way to let your PC's gain money from the "stock market." Namely, if one world/metropolis/country/city/town/hamlet/.etc./ has commodity X at price Z and another world/metropolis/country/city/town/hamlet/etc. has commodity X at price Z+/- then your PC's (and your NPC') can make a profit {or lose profit} by selling or buying commodity X at a smart/non-smart exchange. Every world/country/city/town/etc. should have its own rate of exchange to regulate trade.
Remember: It is up to you to figure out a system, but once you figured it out, it should work out nicely. if you want advice on figuring out a system,ask.