iPhone books: Amazon or Apple?

I just purchased The Girl with the Dragon Tattoo Kindle version ebook from Amazon for $5.99. The same title from Apple’s App Store was $12.99. How can Apple justify a $7.00 difference on an ebook and expect to have a chance against Amazon? Kobo was even cheaper than Amazon at $5.50. Anyway, I am glad to have these iPhone books for travel and not have to carry them.