Even if nothing substantial comes of it, conjecture in and of itself can be fun. I believe the canon's indefinitiveness is part of why the Touhou fan base is so productive; Gensokyo's just such a malleable setting to work with.
If I recall correctly, currency is a concept that dates back to at least to Babylonian times, so it'd really be a more of a marvel if Gensokyo didn't have or develop it. Most of it is probably old human coinage left behind from when they were sealed in, and probably remained in use due to the increased simplicity in barter it provides, already being habitually instilled by the former government. Alternatively it could've been reworked out with whatever system of law that keeps the human village a society
I've been told that Remilia herself claims to be a descendent of Vlad Tepes A.K.A. Dracula, which is supposedly a lie. Regardless, I find that some sort of noble ancestry along these lines is highly likely considering that people who've had wealth in the past, are usually born into it. Therefore it's highly likely that Remilia herself very well may've done nothing at all to earn her wealth and may be living upon the incomprehensibly vast wealth already found within its walls.
If The Dead Princess is making an income, I'd find it likely that it's a highly feudalistic system that's in place, considering that the Scarlet Devil Mansion seems at least vaguely based upon medieval europe. I'd imagine that it's a serfdom of sorts, with Remilia being the equivalent of a Duchess, with her claim over the land automatically indebting a small number of vassals and serfs to farm crops and perform other menial tasks, in turn providing her with resources for trade. She doesn't seem like the sort who'd be adverse to this sort of idea, since she seems to have a small number of people living in her home already in exchange for their servitude already. Sakuya in particular acts as if she had pledged her undying loyalty to Remilia, which is more or less the very definition of what a Vassal is.
This brings about an interesting theory. If Remilia is lying about her ancestry, this could very well be the reason why. Remilia may very well could be a false queen, taking advantage of her vampiric traits to create the illusion of legitimacy to Lord Vladmir's domain. Just a thought.