I read a few years ago that the most expensive book in the world is the Sarajevo Haggadah. At that time it's most recent sale was for $800 million.
The book has a fascinating history and there is an even more fascinating book that relates that history. It's "People of the Book" by Geraldine Brooks. I read it about 7 or 8 years ago and it's one of the most fascinating novels I've read in a long time. Geraldine Brooks, who wrote the novel, was present at the recovery of the Haggaddah, which had been thought to be lost during WWII.
Yeah I'm a bit off-topic but this is good stuff.
Barry