Re: micing a guitar amp
I tried some $30 Behringer mics that were supposedly "as-good" or "better" than the SM57's about 6 years ago or so. Big mistake. They came in a set of 2 with a case for $60. The supposedly "better" Behringer mics did not pick up as much volume, lacked clarity, clipped easier and sounded muffled (almost underwater). We plugged in an SM57 into the same recording set-up and all was great again. We already had SM57's but we wanted more mics so we thought we could cheat the system by saving some money. At least we were able to do a side-by-side comparison.
Ever since that experience I always keep my SM57 handy. There is no doubt you can find cheaper mics and if that's all your needs demand, by all means, go for it. If you want a mic that will last you a lifetime, sound great and be more than capable: get a used SM57 from a trusted seller/shop. You won't regret it.