It really depends on the quality, size, and author for me.
There is "industry standard" prices, which pretty much everyone should follow. A quick amazon or barnes and noble search will give you those prices for any particular length and genre in paperback. If you're more than a dollar outside that range for no particular reason, the novel goes in my mental blacklist. It takes a lot of hype for it to get removed from that list for me.
However, there are some authors, genres, or otherwise special books that are hardback that I'll pay more for. However, those are a minority. For those, I suppose my upper limit would be like $100 USD a book.