Thats why you may as well use Addall (or something like BestBookDeal). It searches and sorts by price from Amazon, Half, Abe's Books, and a bunch of others. I dont think I ever found anything to be the cheapest or anywhere near it on Half so I think that place is overrated.
If its a book I want to keep after the class is over I will ALWAYS go for a softcover and/or international edition. My Systems and Signals book was brand-new for $35 shipped, when the US hardcover was like $120.
I can't believe people still have $500 bills per semester from the bookstore. I've hit $200 maybe once and thats not including what I can get for selling them back (tho I tend to do stupid sh*t like flip my new hardcover Diff Eq book for $5).