If you buy their $5 discount card, you get an additional 10% off every order, and most their books are significantly under list to start with. Usually the cheapest prices overall I can find for new textbooks, but if you go to google and do a search for textbook price comparison, you may get a site that will direct you to a cheaper place for a specific book.
Used: www.half.com & www.amazon.com
Unless there is a significant change, the previous edition may suffice for your class, and especially on half.com, previous editions are often $5-10, whereas new ones are about half price. Several of my classmates did this (I wish I had thought of it before buying my books) and they aren't missing anything that the current edition contains.
I've set up a free book exchange service on www.medstudentcafe.com. Register and then post the name of your book and how much you are willing to pay. Anyone seeing your post will be able to get in contact to discuss delivery and haggle about the price. Sellers can also post books they wish to sell. There's no commission and we won't sell or give your email address to anyone if you register.