Excluding textbooks for classes, I bought this for ~$300 (several several years ago as a special treat)
Most of my collection is thrifted/bought from garage sales (for a quarter to a dollar usually) but I do have a handful of nicer ones.
I'd be really curious to know myself. Outside of college textbooks, I don't think I've spent more than 100 on any given book. Probably not even 80.
I can think of some art books that may have run me up into 40-70, like the Yoshitaka Amano book with FF work. Or the NES Ultimate guide from Contri. The Brawl in the Family comics in print probably cost over 100 actually, but then that's also 3 books.
I can think of a couple I've had eyes on that are pricey tho. The Art of Computer Programming from Knuth isn't rare, but a set would certainly be a nice, and costly, symbol of rigor I wish I had. Best Before (Newman) or English editions of Florent Gorges's Nintendo history books seem uncommon enough to be expensive.