As OT mentions, you can build one yourself. The complete Polymer lower, (receiver, stock and LPK) from New Frontier will run you $110 + ship + trns + NICS. DS arms has an upper for $240, BCG $110 and CH for $14. Pmags can be found for $15 if you look. $525 + whatever shipping is from Joebobs on the lower, figure $10.
Add your $ for optics, but $535 is pretty good for an AR. If you want an aluminum lower/LPK add about $110.