I've also thought about this one, and I think part of it is that the ebook market is small and therefore we are paying for set up costs. Although ebooks seem a cheaper format, initially the book sellers have to make changes to their website, train customer services, set up the system (whatever that involves, there must be something ...) etc. We are paying for that. And of course that means that if the publishers are investing in producing new formats there are fewer copies sold and they have to recover those costs.
I don't mind paying slightly more than for a paper book at the moment, particularly as it means I then don't have to pay for storage for my books, but what I object to is when I have to pay extra but the book is of an inferior quality and does not appear to have been properly checked etc. At that point I complain