Well since I was on the 6 year plan....each year was different since I changed my major so much. I would have to say that my final major which my degree is in, was the most expensive. (English). I had a bunch of professors who would only have our books at the local bookstore Sqaure Books and they mark their books up big time. They did this since Barnes & Noble owns our bookstore on campus and they wanted to support their local bookstore. I probably would spend about $500 a semester and when I would go to sell my books back I would maybe get $20 bucks. One time when I was returning a book they told me the value was 25 cents, I was like I'll keep that one!
Check out this article:
http://www.thedmonline.com
Then go down a little and you'll see something about textbooks....I am not good at linking their stories because they usually don't work