The only difference between this weapon and the DOD weapon is that this one is not selective fire. This Colt M4 costs LESS at RETAIL than the DOD is paying for their M4s.
There is a huge difference between a select fire carbine and a one pull-one round sporter...pistol grip & telescoping stock or not.
There is also a huge difference in the profit one shop might be willing to take versus another shop - but you knew that.
Go to the annual National Match at Camp Perry and watch the civilian shooters with AR15s swapping out their commercial parts for MilSpec GI components where possible and legal, everything from GI match barrels to the GI 5-coil extractor springs instead of the 4-coil commercial ones. There's a lot more to the cost of a military contract item than that of the unit itself, including the cost of the inspection and acceptance process [I was an inspector on Ruger Security-Six revolvers purchased for DOD base security guards; one in three failed initial inspection] maintenance and future spare parts availibility contracts, and product engineering and development cost compensation.