I liked what I wrote so much, that I decided to make it into a post on my blog. This is how I do it in my game. The players don’t like it. They have found some people that are willing to buy ancient coins, like the sage who wants to fill in his collection of coins from the ancient empire, to help him piece together a history of its rulers, and he pays well for them. This is not well enough to offset the money changer. There are a few less than scrupulous merchants who take the ancient coins, but charge more.
Dyvers is wanting realism of monetary exchange in his system. I understand the sentiment. In my game, I keep it simple and just have names for different coins. Some coins, like electrum and platinum, are rare and either ancient and out of circulation, or only used in a few current places. Exchange rate math it complex, just google for exchange rates and figure out the formula to convert from dollars to yen and back to dollars. Does the exchange rate fluctuate like in the real world? I like realism, but adding more things to slow down the game while I do math, is just one more complication and aggravation both for me and my players. If you can handle the complexity in your game without impeding the flow of gameplay, then go ahead. As with anything in RPGs if it works for you and you, the GM and your players are having fun, you are doing it right!
Below is what I wrote in my comment to Dyver’s post:
A simpler way is to have a copper = a copper, etc. but in the neighboring kingdom, they only take that kingdom’s coins, so the players have to go to the money changer and pay the fee, say 3% to 10% or more, depending on how honest the money changer is and if the PCs have insulted him, etc.
Wait until the characters get to a small town and are out of local money and no one will trade with them, or only at a price where the individual or merchant comes out ahead after they makes a trip to the nearest money changer. Prices can easily go to 1.5 times to 2 times or more due to the hassle to the merchant to find a money changer. Reliance on gems, jewels, and bullion could get around that. Reliance on bullion/ingots could have other complications. With gems and jewels can the players get the full value out of them in trade?
Also, if the players haul back a treasure from the ancient ruins, there is the money changer, and maybe the tax collector so the local lord/kingdom gets their cut.
For treasures found on monsters, set how much of it is which kingdom’s coin. Is it an existing kingdom, or one long conquered? If the orcs just robbed a merchant train, it would most likely be all coins for the kingdom where the merchant does business. If a merchant crosses between nations, he will have a supply of coins for each nation, plus more portable jewels and gems.
The breakdown of a treasure into 50% ancient coins that require a tax and the money changer, and the remaining 50% split between two or more kingdoms that require a trip to the money changer, and go a long way to shrink the haul. In this case, I would rule the players get the experience for all the coins in the treasure they haul back to civilization, in AD&D, even though they don’t get to keep it all.
There are a lot of ways to boost the realism and make a copper more than a copper without the complication of exchange rate math or a fancy table, or reliance on a spreadsheet at the game table.