Odds are that some Ruger version of an AR-10 wouldn't cost any less...
Very true, their AR-15 clone isn't cheap though I believe they were fearful of releasing a sub $1000 AR since it would cannibalize the Mini-14. It made more sense to keep them out of the same price range, therefore you can do much better than Ruger if buying an AR. (BTW, I'm not anti-Ruger, I have three Ruger handguns.)
I expect an AR-10 clone from Ruger would be expensive compared to ArmaLite or DPMS.