Over in her blog,
aeriedraconia asked why ebooks aren't any cheaper than paper books. And I explained:
Because the only difference in cost for an ebook is the PPB, also known as "paper, print, and bind," and the cost of freight. However, the publisher has the additional costs of prepping their files for a bazillion ebook formats, and having someone check those files, because the last thing you can do is kick it out the door and have it screw up. "I paid eight dollars for this ebook and it doesn't even look right! It has dropped text and weird formatting and what are these fonts OMGWTFBBQ!!!!"
Which is not to say that the ebook isn't slightly cheaper to make, and that savings should be passed on to the consumer. But on a per-unit basis, that's about thirty cents on a mass market paperback (remember, the publisher only sees about fifty cents of that $8 cover price).
Would it make that big a difference to you to see an ebook selling for thirty cents less than the paper book? Probably not. So the publisher takes the thirty cents because their bottom line needs all the help it can get, and it makes no difference to their sales.