Publishers have always thought that when you buy a hardback, what you’re paying more for is the chance to own it on the day of publication. Paperbacks are cheaper because they come out a year later. The reading public, on the other hand, always thought what they were paying more for was the extra physical mass and quality. (Actually, a hardback costs, one publisher told me, only from 50p to a couple of pounds more to make.) So obviously publishers think an e-book, out on the day of publication, should cost the same as a hardback. And obviously the reading public think it should cost less than a paperback. From this difference in perception stem all subsequent horrors.

(via PaulCornell.com: The Twelve Blogs of Christmas: Ten)

I thought this was interesting - I definitely think of it as a weight related cost rather than a time related cost. There are lots of other interesting points in the blog post as well. Worth a read if you're interested in the eBook domain.