Shure is running a holiday $10 rebate deal on the SM57/58 if you're interested in buying new. Used, it depends greatly. These mics have been around forever and tend to take abuse well, so you see them in all sorts of cosmetic condition. I'd get one of the more beat up ones for less money from a someone with a lot of positive feedback. Best bang for buck
Definitely found a great 57 on eBay for $45 once. Keep your eyes open for people who list under "Buy It Now" - a lot will list it too low, and if you find it first, it's all yours