But either way, it's still cheaper to buy used books, even if you factor in the cost of the Kindle books being free (don't forget, you don't own them), the actual cost of the device will take you awhile to recoup the savings.
I recouped the cost of my Kindle within a few months. The special offers that I took advantage of saved me $40-50, and the one free book per month that I get as a Prime member saved me the rest over the past year. I've probably made a slight profit at this point.
Kindle book prices tend to generally be lower than buying actual new books. I'm sure used books can possibly be cheaper, but finding and going to a used bookstore isn't convenient for me.