I just built a Stag Arms M4orgery. I bought the receiver in a group buy for a 100 bucks. I bought the M4 upper kit (included everything else, even a 30 round magazine) from legal transfers (an AR15.com dealer) for $620.00 delivered to my home. Total cost: $720.00.
I had to buy three pin punches ($21.00 from the local industrial supplier) to put the thing together. Had I planned ahead on the pin punches, I would have just bought a complete pin punch set from Brownell's when I ordered the rifle.
I used the instructions off of Ar15.com and taking my time, cleaning, lubing everything, it took me all of 30 minutes to finish building the lower.
The rifle has a chrome lined barrel, an ARMS #40A2 rear sight installed, an enhanced bolt carrier and overall, I've been pleased with it. I do recommend you restake the bolt carrier gas key per instructions on AR15.com.
I don't think you can build a good quality M4orgery cheaper than what I did mine. You might save a little money if you buy an unassembled upper kit, but you'll spend that on the tools to put the upper together.
Regards,
Dave