You *probably* want two of the same kind that are sort of matched (not necessarily perfectly matched) so that you can do stereo recording. If you're sure that's not important, than one's OK, but most folks (me included) seem to want to record
acoustic guitar in stereo.
Do you mean like something you can run out and buy new for $100 at will? Or do you have time to wait/hunt/stalk and find a good deal on used?
The reason I'm asking is that I have two mics that I use a lot for this (Oktava MC-012) that I got for $25 each recently at a local pawn shop, but you'll find that the price is higher for a used pair on ebay (usually above $100). Another one of my favorites, although I paid substantially more for two is the Shure SM81. A lot of people here like the MC-012 (or MK-012 - same mic, essentially) better than the SM81, but I like both. If you see either of these used in your price range, you should pick it/them up. These are cardioid pattern small diaphragm condenser (SDC) mics that require phantom power in order to work (the phantom power requirement will be there for just about any mic you want to use for acoustic guitar). (capsules are sometimes available for both that allow for an omni pickup pattern, and the MC/K-012 has capsules for other patterns, diaphragm sizes, too).
Anyway, file that away and keep your eye out. For mics you can get via mail very promptly, check out the Naiant series (
http://www.naiant.com/studiostore/microphones.html ) - the guy who makes them is a regular here, btw. These are also SDC mics, and they have an omni pick-up pattern, rather than cardioid. I have two of the X-Qs and I've made some very nice tracks with them spaced out on either side of the guitar in front. It's nice to have both those and the others I mentioned, so maybe consider going for the Naiants now, and patiently waiting for a good deal on the others (if you feel like you even need more mics, which may not be the case).