stfu, guitarn00b!
...
Actually, yeah, you're right. As far as bang-to-buck ratio for recording amps, you really can't beat an sm57. But that assumes you have an audio interface/preamp/mixer/something with mic inputs. The mic may be just $90, but if you don't already have the ability to record it, getting that will likely cost at nearly as much as the mic itself, if not more.
Though honestly, unless you have a REALLY nice amp, I'd spring for modelling software (ie guitar combos, amplitube, guitar rig) before I'd spring for a mic if all you're gonna use it on is guitars.