It seems like, when creating a fantasy universe, the default currency tends to be the generic “gold pieces.” Of course, gold pieces have been pretty standard currency in our world all the way back to at least the seventh century BC. It’s said that King Croesus of Lydia (as in “as rich as Croesus”) had coins made from electrum, a mixture of gold and silver. I don’t really know how much value these would have had when compared to modern money, though. In video games like the Dragon Quest series, there are high-end weapons and armors that cost in the tens of thousands of gold pieces. Does that mean someone buying one of these things actually has to count out thousands of individual coins? And how does the shopkeeper test for purity, to make sure the customers aren’t cheating him? So perhaps there are larger denominations that we just don’t hear about. It’s hard to say. Another question is why monsters would be carrying around coins. Or do they just turn into coins when they die, in some sort of bizarre transformation?
In the early English translations of the Final Fantasy games, the money was simply referred to as “GP.” Starting with FF7, however, they started using what I believe was the Japanese name all along, gil. Perhaps this was derived from “guilder,” which is the currency that the Dutch used before switching over to the euro a few years ago. “Guilder” is actually derived from the Dutch “gulden,” meaning “golden,” so I suppose there isn’t too much difference between that and “gold piece.” It sounds better, though. The in-game explanation in FF4 is that gil were named after the ruling Gilbart family of Damcyan. Gil coins apparently come in different denominations, and some of them are shown as having holes in them, like Japanese yen.
In the Mario games, there really isn’t a name for the coins, and I’m not sure what they’re made of is ever specified. Generic coins look like gold, but SMB3 has silver coins that apparently have the same value. In the old Nintendo Comics System, coins were referred to as Koopabits, which would make sense for Koopa currency but not so much for Mushroom Kingdom currency. The general rule is that one hundred coins will grant an extra life, but even this varies somewhat from one game to another. It would seem that the value of a coin in the Mushroom World is pretty standard, but there are jokes about currency exchange in Mario & Luigi: Superstar Saga. A Beanbean Coin is apparently worth about a trillion Mushroom Coins, which suggests that the Beanbean Kingdom has a much better economy, but maybe they just like to mess with foreigners. I haven’t played Super Paper Mario, but apparently it has a character named Footsteps of Coins who claims to hide the coins that Mario finds in the games.
The Zelda series has actually had a standard currency since the beginning. The rupee is a real-life currency used in and around India, but in Hyrule they’re gems. Different colors of jewels are worth different denominations, but which color has which value isn’t always consistent from game to game.