I don't know of any condenser mics under $100 that I would recommend for your purpose. There's a chance you could strike lucky and find a decent SDC on Ebay or other second-hand sources, but you would have to find someone who didn't know the value of what he was selling and gamble that no-one else in the audio business had seen it.

An SM57 does a respectable job on acoustic guitar.

My first criteria in purchasing microphones is to purchase them in pairs. I like stereo. You like stereo. Lots of people like stereo. So you need a pair of matching microphones for stereo.

Good dynamics should not be passed over.

There's nothing wrong with using 2 of the same microphones on 2 different sources. Voices don't sound like guitars & guitars don't sound like voices. Although 2 diametrically opposed microphones can make for extremely interesting stereo imaging & perspectives. Largely because of their differences.

You might think to put on a small diaphragm for the vocal and a large diaphragm for the guitar? But you may find that the opposite might provide a better sound than what you envisioned? So just because you think it should sound a certain way doesn't mean it will.

Always experimenting
Ms. Remy Ann David

I recently picked up a stereo pair of MXL 604s on eBay for $150-ish. They're no SM81s, but they've sounded great on guitar so far. I've yet to give them a piano run (or mandolin/ukulele/etc). Look into those for some budget microphones (and check out the Tape Op boards to find some mods).

I agree, though, that you can use that same LDC on guitar as on voice, but if the only instruments in the mix are the guitar and voice, you ought to double track the guitar and pan left and right to fake stereo or something of the sort.

I've heard that stereo recording on guitar can sound better in some people's opinion, but that really can't work for me. Namely because my Tascam audio/usb adapter can only run two mics at once, and I want to be recording vocals and guitar simultaneously. I haven't been recording very long (really only messing around) and I'm just looking to produce the best sound I can with what I have.
The double-tracking on the guitar's a good idea though. It wouldn't lay over perfectly, of course.

I know my situation isn't ideal, but I can't be spending more than $100 on this. I'm a college student and I just don't have the money.

Basically I'm asking if there was one mic that you'd advise buying in addition to my SP B1 for my purposes--and under $100--what would it be?

The SM57's in my budget. Would that be a better choice than a $100 condenser?

The thing is, I doubt that anyone is going to name a "no questions asked" good condenser mic for under $100. That's not to say that your SP B1 doesn't sound good on your voice and your guitar. It's just that most people who have tried a lot of cheap mics find that they don't work well in a variety of situations. If you like your B1 on both guitar and voice it makes sense to buy another. But if you are going to but another $100 mic without an extensive tryout, I'd buy an SM57. As you get more money, you might choose other mics for guitar and vocals, but most people keep a 57 or ten around even if they have $10,000 mic cabinet. Can't say that about a B1. (A 57 is great on guitar amps, and drums. It works well on instruments for live sound support. And it can be the perfect fit on some voices and some songs even when far more expensive mics are available. Very good $100 long term investment.)

That's reasonable. Thanks for the info.