As most of you know, I have been a Robert Jordan/WoT fan for all of my adult life. The man could definitely write. One of the things that has always fascinated me about his world were the money systems, and the way coins worked from realm to kingdom.
As much as that promotes realism in a fantasy world, it gums up a game. Ever try to keep track of Gold Eagles, Gold Dubloons, Gold Medallions, and Gold Cogs, all of which have a different weight and value? Probably not, and not just because it is not fun. It makes keeping track of a character's wealth almost impossible.
Here's the problem, though. The Megadungeon is eons old. Its older than the elves, and the dwarves. So there are old coins inside of it. They will be minted with kings images long forgotten and longer dead. Gold, however, is undying. It has driven man to gain wealth for as long as we have had the concept.
How then, should I handle the problem of ancient coins coming out of a dungeon and being stuck into circulation as currency, when the country does not accept that coin? I have decided that most gold, silver, copper, and platinum coins will be confiscated upon entry to the Adventurer's Guild. The aristocracy will re-smelt the coins into a usable, legal, currency. The finders of the gold will be given coins of the realm in exchange for the coins of the past. The city will take a 10% cut of this exchange, though.
Here is the basic exchange rate of the world:
1 silver piece (standard coin used day to day)
10 copper pieces = 1 silver piece
1 gold piece = 10 silver pieces
1 platinum piece = 100 silver pieces