Is there much difference when college text books come out with new editions?

I'm taking a political science class and the book that is required is $120 dollars at the school bookstore I found a girl who is selling the same book for 20 dollars. The book store's book is the 10th edition and the used book that I want to buy is 8th edition would their be much of a difference in the material text?Is there much difference when college text books come out with new editions?
There could be a great difference. As new editions come out, certain things will be added or changed inside the book. For example, I'm taking Biological Anthropology right now in university, and my professor said that Chapter 13 will be vastly different in the older versions of the text, because obviously as scientists discover new information they must include it in the new editions. (You could always inquire from your prof. or T.A.)Is there much difference when college text books come out with new editions?
There might be new things added to it, topics may be reorganized. Bring her to the bookstore with you so you can compare the book she's selling to the one at the bookstore just to be sure that not much has changed.