I would've expected the ebooks to be cheaper. They don't require physical manufacturing, packaging or home delivery. They probably do incur Amazon a persistent charge in the form of keeping them stored in their servers and keeping track of who bought which book, so the buyers can re-download their books. They could very well expire the re-download for a purchase after a certain amount of time. But it is possible, the lifelong storage of the digital books management of their ownership, downoads etc. make an ebook more expensive for Amazon.