I might spend as much as $100 all at once on an RPG if…
1) I’m absolutely certain it will get played at some point, especially if it will get played more than a couple of sessions
2) I’m certain I will derive pleasure from reading and possessing it (see: Fall of Magic)

I think the only way I would go higher than $100 is if there were some kind of experiential component (say, New World Magischola) and/or it was certain I would play the game a LOT (10’s of sessions).

On the other end though, lord knows how much money I have squandered on $5-$20 PDFs here and there I knew even at the time I bought them that I would never play and would probably only skim for 15 minutes. I’m trying to be more judicious about that sort of thing.