I might fall down the figurine rabbit hole, which would collectively add up to way more, but the single most expensive, completely frivolous thing I can see myself buying would be an expensive art tablet with a built-in display. A really, really good one. Which would be frivolous for me because I'm a terrible artist and if I was already wealthy then I obviously wouldn't be drawing for money.
Well, that's what comes to mind at the moment, because right now I'm trying to decide which of a few in the $200 range would be the best.
Oh, I suppose commissioning a bunch of art would count as frivolous, too, wouldn't it? Because I might do that.