If your classes are using these books as reference without homework problems or external media (CDs, Online material), the 6th edition of Comparative Politics can be bought $2.30 used from Amazon and the 13th edition of the Management Science book is less than $20 used. Older editions of textbooks usually contain roughly similar content with some older information and/or different problems.
If they are using problems from the book directly, the 7th edition of Comparative Politics is on reserve in the library and the 13th edition of Management Science is as well.