I've purchased unregulated 80% lower receivers (they're only legally "a gun" if more than 80% of the machining work is complete. At or below 80%, they're just considered a chunk of metal.) and machined them using nothing more than a woodworking router, a hand drill, a jig, and a vise in about 30-45 minutes.

The jig is the most expensive part, costing about $160 or so, but once you have it there's no wear parts and you can use the jig to build as many as you want.