Around $800 for a non-sighted AR15 seems to be the going "inexpensive" rate for the complete rifle. The only option I can think of, off the top of my head, is perhaps a "kit".
The cheapest and easiest option for many people is buying a kit, i.e. everything but the lower receiver - Complete upper, buttstock assembly, and pistol grip. From good, quality manufacturers like Bushmaster, I'm sure a 20" Flattop (V-Match, etc) shouldn't run too much more than $600. From Here, a lower and necessary parts from Olympic Arms, Rock River Arms, etc should be about $200 more. This overall figure adds up to what you are being offered for a complete flattop rifle, which means you are getting a fine deal on a fine gun.
There are many ways to lower prices, i.e. gettin the lower, parts, buttstock, trigger, upper receiver assembly, and barrel all in different shipments. While this might cut off a small % of your total price, the hassle of assembling everything is sometimes not worth the few bucks you spend buy buying a complete gun, or just snapping a new upper onto a new lower and having the same result. You should be impressed by the overall quality of this bushmaster (I would recommend the 20" if you plan on mounting optics, as IMHO flattop AR's are designed for longer-range "precision" shooting over Service rifle AR15's, and a 20" will be preferable over a 16" in this case).
[i]Anybody else?[/i]
Jewbroni~