Ah yes... Blablabla coin is worth this and that yen, even though exchange of any kind can't be done. When Rowling did magical coins exchange they had a purpose, no matter if the rate was flawed, there was an actual exchange of coins from one to another. If the other world doesn't have any kind of contact, there is no way to make any guess about how much would one coin relate to the other, they have no relation. They could try to compare buying power of street vendor food or one night in a hotel/inn, but not coins themselves. That would give some approximation, but still leave other things outside that range, like buying a sword, armor, or a horse for example. No way anything can be lumped like that with a fixed virtual exchange rate.
What these novels should show instead is how much is buying power of one gold coin: Could you buy a cow with it? Perhaps it's enough to buy you one night in a brothel? Just anything would do instead of bland it's equivalent to xxx yen.