Mmm, I heard of Muenzuka, and I am pretty sure there are other places humans creep in sometimes. But I wasn't sure if there might be...interdimensional smugglers, for want of a better term.
Not to be overly analytical, but gold or silver was preferred as a currency because there was no central bank to insure paper money's value. Coins of precious metals have self-evident worth, and is probably what they would default to. Though it would be funny to see someone minting coins, considering how old Gensokyo is, chances are they just use Meiji and pre-Meiji era coins. I wasn't sure if they may have specified in a manga or not.
What provoked this chain of thought was actually the first Touhou content I ever played to completion. No, not a game or a fan game. Its a mod for Mount and Blade called Gensokyo Warfare. In that game, P-Points are used as a standard currency. Though Outside World currency is considered far rarer and more valuable, to the point a 2000 Yen note (about $14) provides Cirno with life-saving surgery. Its a good McGuffin for the plot, though I'm still not sure why the natives of Gensokyo would care about Outsider money. Dreamers like Maribel probably can't take money with them. The Lunarians have no interest in trade with Earth, and I doubt Yukari shops at Cosco. It would probably be a novelty collector's item at best.
But it did raise the question to my mind.