At the beginning of every semester, I compare prices among the three campus bookstores. T.I.S. is almost always the cheapest of the three stores.
However, I can usually find all my books for much cheaper on Amazon.com. My Discrete Math book cost $125 (used) at T.I.S., and I found it for 80 cents on Amazon.
My recommendation: email your professors before the semester starts to ask what books you'll need. Order the books on Amazon or eBay.