Textbooks from the college bookstore can be expensive. Even the used ones can easily cost $50. One great source I found online to comparison shop is ISBNDB. When you enter your textbook's ISBN number into the search engine, this site pulls up the lowest prices from dozens of online bookstores (such as Biblio and Amazon). It also collects the best price from eBay and Half.com.
I recently bought a collection of essays for my college writing class for $2.49 from eBay. It cost $34 used from the bookstore. I asked my professor if an earlier edition of the book would work, and was able to cut costs considerably when she said yes. Earlier textbooks usually aren't fundamentally different from later versions, and it pays to ask your professor if an earlier version will suffice. It can cut the cost down on used books by half or more.