Not looking to spend a lot, maybe $50, but I have no idea what the costs are for what. Any suggestions?
I agree with those who said headphones.
But is it typical to get two monitors to make it stereo, or do you usually just have one?
Stereo is of more interest to some people than others. Especially in live performance (not your situation, I know), lots of people go mono, for a variety of reasons. Regardless, if budget is tight, it's better to get one decent speaker than two crappy ones.
I think the B208D is surprisingly good for the money. It's not going to push much deep bass, and it's not great for acoustic piano sounds, but for most sounds at moderate volume, not bad at all. I think it's a good budget choice, probably the least expensive piece I would recommend.
The B208D is cheaper than the CM-30 Roland Cube, which I have not heard. I don't know which would sound better at lower volume, but it's a pretty safe bet that you could get more out oomph if you need it out of the Behringer. As people here may know, I don't put too much emphasis on spec sheets, but the specs here are different enough that I think you can draw reasonable conclusions in this respect:
The CM-30 is 30 watts into a 6.5" coax speaker. They don't say whether it's 30 RMS or peak.
The B208D's is an 8"+horn bi-amped system, rated RMS at 135 watts to the woofer and 38 watts to the horn (160 and 42 peak, which is where they get their "200 watt" claim from). Max. 113dB SPL@1m. (No similar spec provided for the Roland.)