I still think it's very possible... it would just take some effort. Just in the few minutes I was looking right now, I found a complete upper half, including BCG, for $600 (bushmaster) http://www.ar15.com/forums/t_7_159/1...MEGA_Rail.html
This one people are avoiding because it was spray painted. I realize that this is not quite at the '$700 gun' price point, but it's getting close... maybe the seller is motivated.
If I could find a more unfashionable upper, for example with a fixed carry handle, or a DPMS extruded upper, it could be done for less, I have no doubt. You just have to put some effort in it, and move quick.
Palmetto has some uppers back in stock from time to time, I bought a brand new upper with a hammer forged barrel for $400 a few weeks ago, but finding a BCG outside of an upper that's not overpriced would be the trick right now.