How much would you pay for a book?

How much would you pay for a book? Or, to put it another way, how much is a book worth to you?

A couple of weeks back, I was chatting to an acquaintance about books. During our conversation my acquaintance stated that many ebooks are overpriced (he mentioned that some cost £7 or more), and given the low cost of producing electronic versions they should be more reasonably priced. He also went on to state that he bought many of his books in charity shops, with many retailing for as little as £...