First, I must admit that I'm a roadie. Mock away.

.....

That done and out of the way, I'm using my road bike to commute, and I'm afraid I'm thrashing the poor thing; thin aluminum tubes are definitely not fond of ill-maintained city streets. I want to put together a fixed gear bike, but I'm on a hella small budget. Could I scrape one together for under $300 US? Less?

If you were going to do it, how would you?

TIA

thrift store or ebay frame/bike
surly track hub/MA3 rear wheel.

a suzue basic hub might be cheaper than the surly, but the surly will last longer, IMO.

ebay helps if you're a smaller rider, because the smaller (48-52cm) track-specific frames tend to go a lot cheaper. there's a seller "applecart" who sells a lot of pretty decent converted road bikes for cheap. they don't usually have track rear wheels, so you'd still have to get that.

one thing to look for (besides horizontal dropouts), for sure is the cranks, make sure the spider isn't separate from the crankarm (like the pressed together kind). you want a crank with the spider and crankarm all molded/machined from one piece.

Click here for a story on how I built my first fixed. I spent a little over $100 total as I recall because I bought new wheels and tires.

If you can be patient and search, you can build one for cheap.