EP1990, the post by z-michigan covered it for you.
All other posts have interesting sidebar information, none of which helps you,
As far as anwering your actual question. The Internet is funny that way.
As you already know, these rifles are not like the AR15 modular series, where most of the industry is on the same page. For the 308 platform, RRA is probably as different from the rest as possible. It will be very difficult to proceed and succeed, but you may get lucky and find the lower you need. Don't forget about magazines.
Earlier this year I built a current Armalite type and a DPMS type. Both types share some parts with the AR15, so I saved on a few items. The 16" DPMS type with carbine stock cost me $780 to build, the 20" Armalite type with fixed A2 stock cost me $770 to build. Each only has a few Armalite or DPMS parts, most are no-name parts from various suppliers. The 16" is fun to shoot but not accurate at all. The 20" has a genuine Armalite barrel and is quite accurate, and easier to shoot.
Mostly due to the lower, it would cost 50% more to make an RRA clone in either configuration. But you can do it. Be diligent if you want to make that goal.
And please ignore those who tell you just to buy a complete rifle and never look back. They simply don't understand your personal goal, and the value of planning and assembling your own rifle.