Garage sales can fetch you books for as little as $0.10 each and the Library book sales $0.25 to $1.00 each. A small library near our home is currently selling used books for $1.00/bag. They're older reads but still good. I bought a couple bags and shipped them to our adopted soldier in Iraq and am reading a few myself.
Our local thrift store has books half price every Tuesday. Last time I shopped there, I bought 10 brand new Dr. Seuss books that had never been opened for $0.50 each.
My last purchase at the library's used book store, I spent $16.00 and got the huge collection of books below. If our darling son wasn't so fussy in the store at the time, I'm sure I would have bought twice that much.
What we do with our books when we're done with them is donate them. Here's the general criteria we follow:
OUR LOCAL LIBRARY RECEIVES
~ All perfect condition hard cover kid's books.
~ All our son's outgrown, once loved board books.
I check the online catalogue first to see if they have them and if they don't, or only have one copy then I donate the book.
OUR DAUGHTER'S CLASSROOM LIBRARY
~ All paperback kids books get donated to our daughter's classroom library after she's read them. This is a good incentive for her to read and I get her to sign the inside of each book before it's donated.
OUR DAUGHTER'S SCHOOL LIBRARY
~ All other hard cover kid's books that may have just a few marks in them get donated to my daughter's school library.
This works for us on many levels. We get to purchase, read and enjoy the books then pay it forward when we're done with them. Our library and school greatly appreciate these donations.