Found a good article that talks about both sides of the argument that a Kindle can pay for itself. On the one hand this could technically be true. If you buy a lot of brand new hardback books, then the Kindle will eventually save you enough money to pay for itself, since e-books tend to be a few dollars cheaper than their printed cousins.
But several recent surveys have shown that those with e-book readers both read more, as well as buy more. So you might technically be saving money, but you’re also spending more as a result. So it’s kind of like when a spouse tells their partner that they should be happy because they’ve saved lots of money shopping. Technically true, but at the end of the day your wallet is pretty empty.