My bets on a complete lower.
You can build one for under $300 Vikingdad. Get a stripped lower for about $100, lower parts kit for about $50, and a complete buffer tube and butt stock for about $60 to $100 (depending on what kind of stock you want this can go up to $300). On the low side this is about $200 before taxes, shipping and dros.
BUT you should already have the buffer tube, end plate, castle nut, buffer and spring, and maybe even the butstock do you can save even more. You may only need the lower receiver and lower parts kit.
To stay legal and read up on all the gun laws in Cali, I highly recommend visiting, www.calguns.net. Its a great resource for all of us that live in Kommiefornia. There is also a sticky at the top of the centerfire rifle forums that has all the local gun shops and ffls on a map.