When I got my Sony Reader PRS-600, I knew I wasn't going to be spending money on eBooks. The device was to be used exclusively for free reading material. Meant to reduce the costs of my reading habits, it was hoped that the device would "pay itself off" within a reasonable lifespan.
Assume all books cost $12. They don't, but go with it. The device costs $300 (it seemed so much cheaper back in October, but now Sony is much more expensive than competitors... that should change). 300 divided by 12 equals 25. Now look at books costing $15 (close to reality). Now the number is 20. Basically, somewhere between 20-25 free books read on the device "pays it off". And so my silver Sony has given me my money back. Cool.