I love using books as rewards for my children. I have been to the dollar section at Target and Walmart on several occasions, but even dollar books aren't all that cheap!
Although I am usually very particular about the condition of our books, I am getting used to the idea of second-hand books. On my most recent visit to our local library, I stopped by their book sale area. I was able to find several books for only a quarter each, and I could have even filled a paper grocery bag full of books for only $2 (if I had more time to browse, of course).
Check with your local library to see if they have a book sale area. Many libraries even have a 2-3 day sale event during the year when they sell hundreds of books to raise money for programs they offer. Just as a little warning...these books typically show some wear and tear, and you will definitely want to wipe down the covers when you get home.
Have you ever taken advantage of a library book sale? Where do you go for inexpensive books?