I'm new to the world of ARs, but this deal looked really good. It's a complete mid-length 16" upper and complete LPK from PSA for $650 + $16 s&h. Even has an MOE stock.

http://palmettostatearmory.com/index...ctr-rifle-kit/

PSA also sells their lowers for $120 or $100 for a cosmetic blem model.