Cheap and AR10 do not go in the same sentence. Hate to tell you bluntly however, this is the realization for a non-standardized platform.
The AR15 platform is standardized, so therefore, many companies make the forgings, sell them or machine them themselves, and sell to the general public, and the prices fluctuate accordingly.
You essentially have two systems, with variations from these two:
Armalite - AR10
DPMS - LR308 (RRA is similar, not exact).
Knights Armament - SR25, which the DPMS and SR25 can utilize the same magazines, only similarities there.
Back on topic. For the stripped lower, you'll need to know which system you plan on endeavoring in. You choose from the above two (not KAC, as they're sold mostly as a complete system, ie. rifle). Once you choose the platform, then you can pick to your hearts content on makers whom machine to those specs.
Next question: Do you have an upper already being built, or already ready to shoot that you need the lower for? Or starting with the lower and going with an upper later?
If I may suggest the next issue: type of machining. There's forging, and Billett 7075. Most AR10/LR308 will be forged, and you can buy the respective manufacturers' parts kits. However, these are still as costly as some billet manufacturers. Specialized Dynamics is supposedly (correct me here) coming out with one, Quentin Defense is as well this year. I currently use an SI Defense, but with Crusader Weaponry's name on it. Others come to mind as well. Mega Machine and SI Defense are the best competitively priced MATCHED sets available at this time. You can however, buy from Rainier Arms' website the Mega MA-TEN lower separately if in stock separate from the upper receiver. These can be found for less than $400. POF, RRA, and DPMS sell them separately as well. The costs for these are higher, as they'r enot part of a complete rifle.
My suggestion, is to buy a matched set if you plan to build. otherwise, purchase the entire rifle if you have the dough to do so.
I build all of my rifles, however, if I were to PURCHASE one outright, I'd start from there. Hope that helps.
--- On a side note, my AR10/LR308 from SI Defense is a good product. Mega Ma-Ten is selling like crazy so it's obviously a good platform to work with as well. Very intuitive design features there.