Wow, I think I’m doing what you’re doing except the SS barrel (cost) with maybe a couple other goodies such as adj. stock, flash suppressor, etc. and my quote is twice your $600 figure.
Are you going though a gun shop or a private dealer that works gunshows?
I can send you the name of the outfit in NC that built my lower but you may have to find somebody in your state because the lower is the FFL component.
If you have a business, even if it is real estate, Brownells will allow you to register as a shop and give you a dealer discount which saves between 10-30%.
Also you have to do a lot of the labor yourself but the AR platform was designed so the low potential low achievers could field strip the system during combat.
I figure in round numbers:
$200 Complete lower (stock & trigger group)
200 barrel + gas block & shipping
100 upper - populated flat top
130 complete carrier bolt
40 forend
20 charging handle
20 gas tube
10 Magazine
$690 based on prices from Brownell, M1, and Yankee Hill Machine (http://yankeehillmachine.com/store/uppers.html)
then you will need some sort of sight, including rings, or iron sights if you go with a carry handle.
but you can beat the above prices at gun shows and eBay, except the lower. However, you can buy a complete DPMS AR 15 for about $600 new at the shows but if you build it yourself you can use better parts.