If you go to Goodwill  you can find used books really cheap. I have bought Rod & Staff, Abeka,  Public school text books. anywhere from $.49 to $.99 . I just bought   some Abeka books for  little or nothing.  Some have never been used or might have a few pages missing.

Your goodwill must be better than mine.  The Goodwill in my town just mostly sells outdated and worn out books.  And they put a high price on their books like $2.50 for an old, old  obsolute textbook.