International editions, man. $120 textbook? $30 international edition. Same book, just printed cheaply and bound in softcover. Exact same text and problems.

Example:

Domestic.

International.

A warning, however, some international editions, while the material is identical, some problems may be changed. I have an international edition of Optics by Hecht and half the problems are identical to the domestic edition, while half have been changed. And I mean ENTIRELY, not just minor variations in the numbers. A quick look through the preface revealed the phrase "Most of the problems at the end of the chapter have been suitably modified or replaced to mirror the type of questions posed at Indian university examinations."

Also, as has been mentioned, you can often use older editions. I took a real analysis class and had the old edition of the text, and the instructor let me photocopy the problem sets from his book.

The textbook publishing industry is fucked up, at best. Often new editions just contain re-written problems in order to sell a new batch of books to make money for the publishers and authors, while the core material remains identical. It's a bitch.