Non-fiction: $20 - $100.
Fiction: $1.

There are the rare cases when I am willing to pay more for fiction, but that is very rare. When it comes to fiction, I'm cheap because it is entertainment and entertainment is easily interchangeable. I will happily read a cheaper book from a competing author or even choose other forms of entertainment. It should also be noted that I'll pay those prices for print books. Fiction ebooks tend to be worth less in my mind since they are ready for the trash bin after being read once.

I will pay dearly for non-fiction though, since I actually get something from it. I am willing to pay $20 to pursue a personal interest at an amateur level and $100 for professional level books.