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.