Not certain how other school bookstores function, but at the end of the semester, typically the unversity bookstore would sell the last year (one generation older than the current version in use) edition of a number of texts for $1.00 to $2.00. Built up quite a library of mathematical reference texts by taking advantage of these sales, at a fraction of what text books typically cost (and have kept nearly all of them as references).
In several cases, the text would end up being the same used for an upcoming class. Would specifically not purchase books for this class, and then ask the professor (either after the first class, or the first opportunity during office hours) if the older edition of the text would be acceptable. In nearly all cases, he/she would indicate it would be, and a number of professors even offered their notes on which exercises/examples would be the suggested homework (one even photocopied their entire set of notes in reference to that particular version of the text).
In short, would definitely advise asking the professor.