Buy a stripped lower, then order a parts kit
I got my lower from Lonestar Wholesale, and my "kit" from MAPartsinc.com, shipped in 1 day!
My lower was $120(Armalite/eagle), and 16" midlength, 1:9, flat top upper, shorty LE stock, and green furniture was $515(shipped)
I also bought a carry handle($35-100)
So, $670-735ish for a complete rifle.
If you just want something cheap, CMMG's bargin bin is great, $570 for a rifle(usually DPMS builds, or used to be) +$25FFL and shipping, not bad...