Get a parts kit and stripped lower.
This is how I built my first AR. Total cost to build an operating AR-15 was $395, which is alot lower than the $595 for a Plinker. You can assemble a brand new kit on a Brand New Stripped Lower for $495. I used SP1 Used Parts from a Pre-Ban for my $395 AR. Model1Sales sells brand new kits for $400. Lowers were $95 from Gunsmoke when I purchased mine. Guns-N-Stuff has Cast Oly Arms lowers for $80.
It is real easy to build except for installing the pivot pin detent.
Whenever Tapco brings back its M16 Kit, the price to build an AR should drop below $300. I figured out that with the Tapco kit, you could sell the M16 parts for exhorbitant prices on E-Bay and buy AR-15 parts cheaper. The result would be a $250 AR-15. But, you would need to either sell off the M16 parts or demill them. You also would need to not have an AR-15 Receiver on hand.