I bought a nice Armalite stripped AR-15 lower at my local gunshop recently and installed Rock River Arms parts in it to complete the lower assembly. After that was done I ordered a complete 16" A4 midlength upper directly from Rock River's website. Putting the upper and lower together to make a complete weapon takes about 15 seconds. My functional AR-15 with a standard A2 front sight and a Magpul MBUS rear sight and one 30-rnd magazine came in at $815. Optics are optional and extra of course.
Truthfully, the S&W M&P-15's at $599 complete are a pretty good deal and seem to work well for those who have purchased them. Depending on your needs and desires for an AR-15 that might be a good way to go.
The stripped lower:
The completed lower with RRA parts installed:
The completed AR-15 with Rock River complete upper installed: