Acoustic instruments in general are pretty tough to mic. With a budget of $100, you are somewhat limited but it is still doable. I would stay away from USB mics if possible. Do you have an interface that you record through?I would look at a couple of Behringers, a C-1 studio and a C-3 studio. You might also check out MXL, they have a few condenser mics in that range. If you don't have an interface, consider getting an inexpensive one with at least one XLR and one quarter inch input, and then getting only one mic.
As far as mic placement, the standard is a mic just between the bridge and the soundhole, and usually one at the twelfth fret. However, each guitar is different and will take some experimentation.
Another option is to pick up a little higher end mic and only use one. You really have to experiment with mic placement if you have only one mic, but you can usually find a sweet spot.
Good Luck, and hope this helps.