Yes, Amazon's store is not a good advertisement for the value of electronic publishing vs. paper. It is better than the Sony store, but that is not a ringing endorsement. The price differences are pretty random and nonsensical, but at least they're often in the same range as a paperback book instead of selling it as though it is a hardback even though it has been out in paperback for months. Compare that to Baen books, however. The last book I was just reading is $8 on amazon for the dead tree edition. At the Baen online store it is $6 with no DRM and in a variety of formats. And they've been selling all their books for 1-2 bucks cheaper than the paperback edition for the past several years, even when the book was still in hardback. This is even cheaper when you consider the "webscription" bundle of all the books they're publishing or republishing in a month, typically 3-4 new hardback books and 3-4 older paperback books for $15. If you'll read half those books, that is $5 or less per book.
I've spent around $1000 at Baen's store in the last 3 years to put books on my Sony Reader. The only problem is that they can only publish new and interesting books so fast. I've bought pretty much nothing from any other online bookseller (unless you count the free $50 coupon that came with my Sony ebook reader, where I discovered exactly how bad the Sony store sucked). It's not that I don't want to buy other books, it's that all the other bookstores I've found either had restrictive DRM that didn't work with the Sony reader or had pretty horrible selection. Or both. If I could have found a store like Baen with a wider selection of books, I could have easily spent another $2000 on my book habit. Instead, I reread old books, or look for them from pirated sources in badly OCRed and formatted versions.