Here's a complete upper that recently sold on barfcom for $575...
Agreed about DPMS being low end... but my point is, if you're quick, there are deals out there. I just finished building a few rifle myself on the cheap, but I already had BCGs from before the crazy.
The other good thing about an AR is that you can buy half now, half later, or buy as you can afford... and sell off parts you don't like, or upgrade the quality incrementally and affordably.
An $800 M&P would definitely be a better way to go, if you could find one.