I have a hard time rounding down the materials, printing and labor, transportation, and storage costs to ZERO, though. Gotta be money there somewhere, even if it's not a huge number.
And there's not really a corresponding cost for the ebooks, either. I guess there's a cost to format them for release, but there's already gotta be some format to get it to the printer, can't be much harder to convert to various standards (pdf, mobi, whatever). About 2 seconds and a mouse click on my home pc, imagine you've got dedicated software that handles it just fine.
If you can sell the paper book to the reseller for 25% off, you're still making the desired profit at $6 a book. The extra $2/book is the reseller covering their costs (storefront, storage, labor, whatever) and still making a profit there too. Since THAT'S the real cost that the ebook is eliminating, why shouldn't they be priced closer to $6 than $8? I'm sure Amazon still wants a cut, but there's no shipping, only gotta store 1 copy, etc, so their costs can't be as high...