It is worth noticing that there are difference in the value of money, and the purchase power of it.
Since DF has money made of valuable metals, their value is relatively same world wide: Gold is gold no matter where you are, and thus gold from your Dwarf fortress is just as valuable as gold from Human city (we ignore historical value for now, that obviously changes things but people don't normally pay everyday shopping with historical coins, so...). Since the value of coin is equal to the value of the gold in the coin, if we had two gold coins of same purifty and same weight, they would be equally valuable even if one is from an empire that is on the brink of destruction ad the other was from the stable empire. Historically empires on economic trouble made their coins less pure in secret, which lead to them losing value when the truth came out. If we ignore this possibility, then DF money should have same value no matter the state of the minters economy.
Purchase power is a different thing. Gold only has value, because we give it value. It is pretty, non-corrosive and easy to work. It is also rare enough to be hard to get, but not so rare that it would limit trading with it's rarity.
Purchase power depends on the situation. If you have a wealthy city with 1000 gold coins in circulation on markets, 100 gold coins would be a big fortune.
If, on the other hand the market consists of 100 gold coins and a barrel of water while in desert, the gold is pretty worthless.
However, if you could sell the barrel of water for a man with the 100 gold coins and then get into the city mentioned before, you could use the gold there, and be rich.
This ideally is what trade is. Traders buy things they can get cheaper, and hope to sell them at more expensive price elsewhere. Coins are just a way of tracking the value, or storing the value in easy to carry form since most traders accept gold (again, for it's ease of carry and generally acceptable exchange).
IMO in dwarf fortress money should work like that. There is limited pool of goods to buy, with the money available. This pretty much controls the value. Large influx of coins from other sources should not make those "foreign" coins any less valuable (gold is gold, no matter whose picture is on it), it lowers the value of all coins of that type.
However, this is more limited to amount of everyday goods and everyday money. If there are millions in the fortress vault, but only copper pennies are used in trading by peasants, the one gold coin is valuable in that trade even if it is only a small part of the fortress value. And other way round, if food stocks are full, but un accessible to peasants, the loaf of bread might cost you very much, even if the "fortress" had lots of food. This is more closer to our modern consept of supply and demand.
But on state level (fortress/town), caluable materials have base value based on the total amount of the materials in the world, or within the reasonable area. Again change increases with distance like in middle ages, spices and silk was very expensive in Europe, less so in far east. It would be stupid to count the wealth of far off lands, if there is no contact or regular travel.