Oh, you think so? Well I personally believe in Chekhov's gun, and I think that adding features like a money system half-assedly without making it part of the concept isn't just not useful, it is harmful as well.
The way you see it, the only thing this feature would add is brainless grinding (either in order to get every weapon asap, in which case it is annoying for the dev, or to get a better weapon when the game is too hard on the player, in which case it is a pain for the player itself), especially since monsters respawn. There is not a single good point about buyable weapons if you don't imagine and develop a whole concept around it.
Sure, in order to refrain the grinders you can forbid them from buying some weapons too early in the game, but how would that be different from finding the weapon in the late game without having to buy anything?
To make a money system actually useful, you can force the player to pay each time he uses a refill, or to pay for his missile ammo (or even other weapons' ammo), or to pay for additional health tanks, or to pay for items such as the whimsical star... Then you have to set the prices right so that it isn't abusive nor easy-goin-- oh wait, aren't we developing an actual money system here?
That's my point, basically: as I said before, what you did is good, Carrot, but I want to warn the modders that would be interested in using that for their mod:
DON'T. Unless you have a good idea of what to do with it, more than just Cave-Story-but-weapons-have-to-be-bought mods.
About the <BUY function, yes I didn't mention it as if it was hard to do. But you still have to do it.