Probably apples and oranges to compare the cheaper priced AR's with the Ruger SR556 since there are those who have economics in mind and aren't going to go out and spend the money on a tricked out AR, whether it be the SR556 or any of the others. As a comparison, a huge amount of people bought Sears model shotguns or the like a few decades ago, as compared to those who bought a good Winchester, Browning or Remington (a matter of economics).
A true comparison might be if Ruger were to come out with a "base" SR556 without the features of the expensive model. Then it might be fair to make such a comparison. Ruger usually has ways of cutting to bare bones in order to get a reasonable sell price, so the final price of such a Ruger would probably have its advantages.
Chet15