I agree with Clovery that with some professors, it would be at best, a lot more work to have one edition older and could at worst, be a total disaster from missing/different info. On the other hand, for other professors, it's not a big deal at all and can be a huge savings to buy one edition older.
Using an examination & assessment text by Jarvis as an example:
Current edition, new, list (bookstore price): $109 (no shipping, bookstore)
Current edition, new, Amazon: $90.09 (free shipping). Save 17% from bookstore price. And of course you have the option to sell the book at the end of term for an even lower net price.
Current edition, used, very good: $54 + $4 shipping = $58. Save 47% from bookstore price -- and you have the option of selling it back to Amazon for $44 (minus $4 shipping) = $58 - ($44 - $4) = $18 net cost
One-edition-older, used, like new: $12 + $4 shipping = $16. Although you save 85% off the new bookstore price, a more fair comparison would be the used current edition price of $58, which means a savings of only 72% but one downside to that savings is that you are unlikely to be able to sell it to anyone else. Also, you may not have supplemental CDs and you are very unlikely to get online access (though I have for SOME one-edition-older texts). Comparing the one-edition-older price to the used current-edition, there's almost no savings if you sell back your used current-edition copy for $40 (net)
As you can see there are a lot of non-financial factors to weigh. Are you the kind of person who keeps their texts or are you OK with selling it back? Do you normally find the included CDs and/or online access stuff useful? What is your instructor's exam/lecture style -- is it heavily tied to the current edition text?
When you talk to people 1 or 2 terms ahead of you, you want to find out the teaching style of the professor that you will have (exams based on lecture material or lecture and textbook) and whether or not the supplemental materials are useful (i.e. CDs and online access codes that one gets when buying new, current editions). Obviously if you learn that the instructor includes materials from the text that are not in his or her lectures, you're going to need a current edition. Similarly, for some texts, having the CDs or online access are well worth the price of a current edition text. But then again, depending on your specific professors, it may be possible to get by with one edition older for 50% or more of your texts -- or even sharing a current edition and/or using library books (either traditional check out or reference desk copies).
As long as you're talking to someone one or two semesters ahead of you, might as well ask them about supplemental publisher study guides and third party study materials such as Davis Success series or Pearson (formerly Prentice Hall) Reviews & Rationales series. Some courses you'll find out that the publisher study guide was horrible and sometimes that it's fantastic and well worth the extra cost. Others, you'll find out that a particular third party NCLEX style question guide is particularly representative of the types of questions your professor will have on the exam.
In the end, it's unlikely that anyone on AllNurses will be able to tell you for sure what to do, particularly since no one can post instructor names telling you about their courses/exams to help you with your decision. If you don't know anyone ahead of you, then my personal choice would probably be to not gamble on the one-edition-older. However, if you can network your way into talking to some people ahead of you in the program, you might find out that you can achieve significant savings with the one-edition-older texts and/or other options previously mentioned.