Do an ISBN lookup on some of your books, It will seriously make you wonder what you're paying for..
http://www.isbns.net/compare_prices/9780536082008 - $9
http://www.isbns.net/search/?query=0-536-30874-8 - $5
http://www.isbns.net/search/?query=0-536-75360-1 - $6
http://www.isbns.net/search/?query=0-536-08352-5 - OMG!!! $20
When I took political science and art classes in community college, I was LUCKY if I could find something cheaper than $40. My PoliSci book was $60 used, $120 brand new. I never had the luxury of spending $20 on a book.

