There are other options for searching for used books and purchasing books at a much lower rate than the cover price. Thriftbooks online is a great source for finding used books if you don't have a local used bookstore in your area. You can find most used paperback books for around $3.60 a piece. In addition, for orders of $10 or more you get free shipping. Depending on the used bookstore, Thriftbooks may be more or less expensive. It will also depend on the book.
Thiftbooks also offers a 15% off deal for all first time buyers. Adding to this, if you setup up an account, for every $50 you spend, you get a $5 off coupon. You don't have to spend $50 in a single order. Your purchases are cumulative and add up over multiple orders.
Don't get me wrong, I prefer local used bookstores, but if there isn't one available or the local store doesn't have the books you're looking for, Thriftbooks is another great, cheap option for the book lover in your family.