The best AR under $850 is the one you piece together yourself. My gun:
DPMS 16" heavy SS upper: $440 on sale
Del Ton stripped lower: $120 in a local shop
SS lower parts kit: $55
M-4 4 pos stock: free give away
Total: $615
It took me roughly 40 minutes to put together the lower, and that's never having touched an AR lower before, it's easy. Had I bought this gun in a local shop it would have cost me $1000+ easy. So put one together yourself. In addition to saving you money, you learn the inner workings of the platform. It's a great learning experience.