It's possible to put together a complete lower right now for less than $350. Does that sound like a price point that would work for you? Orion Arms in Jeffersonville has plenty of alloy receivers for $200 and I've seen a bunch sell on gunbroker for right in that neighborhood. Palmetto State Armory sells a complete lower build kit (fire control parts, pistol grip, complete M4 stock assy) for $136, although availability is spotty. I ordered one of those last week, myself.