Borrow copies from the library on campus, take them to the nearest photocopier, and make copies of whatever parts you'll actually use.
I took engineering in school. Probably close to 90% of the things I learned hadn't changed in the last 50 years, and in lots of cases, the 50 year old textbooks taught it better, because they couldn't rely on a fancy multimedia CD or color photos to distract you.
I feel no remorse screwing over the textbook companies when they force students to buy the "new edition" of the textbook. Adding some new illustrations or photos to last year's edition, renumbering/re-wording the review questions, and changing the cover doesn't make it a new edition.
I don't know about the U.S., but in Canada you can photocopy certain percentages of books without even breaking copyright laws.