If you're looking for a cheap way to build up your stash, head to your local library's used book sales. Not only do they have great books for adults at anywhere from $0.50 - $2.00 a book, they are perfect for kids' books, too. When my son was younger, we would buy Clifford, George, and Bob books by the boatload. Now that he's older, we pick up the Calendar Mysteries, Cam Jansen, and Magic Tree House for a song.
To be honest, we are nearing the end of book sale season. They are usually most prevalent in fall and spring, but there is a great, free e-newsletter you can sign up for that notifies you of upcoming book sales in your town.
Check it our here: Book Sale Finder
But, please don't blame me when you need a second house for all of your "new" books! :)