I got Brandon Sanderson's Way of Kings leather-bounds, which were definitely the most I've spent on a book, but my second place is more interesting.

I found a copy of a book called, Our Lost Explorers: the Jeannette Arctic Expedition, at a local used bookstore. Got it for $50, but I've seen it listed for way more online in similar condition. It's the collected accounts of the crew of a ship lost in the Arctic, as well as everyone that went to look for them, and how the survivors eventually made it home (on foot). It was released in 1882, like, as it was all happening, the book is illustrated, and the cover is hand painted. My copy originally belonged to a school