It's worth taking advantage of deals that aren't "half bad." Five or six years ago, ArmaLite had an introductory deal on their Eagle Arms polymer (manufactured by Cav Arms) lower receivers: $125, including a matching lower parts kit. Not too long after that, I got an ArmaLite SOF upper assembly (16" chrome lined barrel, mid-length gas system, etc.), complete with bolt/carrier and charging handle, for $379 on clearance. Total price for the carbine: $504 (plus FFL & shipping, of course) - not "half bad!" It's worth watching the ArmaLite website for sales & clearance items.
That’s where the Plum Crazy lower comes in handy on a sub-$500 build. It is around $125, and it is a complete lower with all internals, buffer and buffer tube, and adjustable buttstock. Literally lock in an upper and you’re ready to go.
Except it’s all plastic except for springs, buffer, and bolt catch. The rest of the lower, oncluding the hammer, sear, trigger, safety, mag catch, and even the take-down pins are plastic.