you can't get the flush cups in the receiver anywhere else either, or the rounded bolt.
i'm not saying those features are worth it for everyone. i believe the design goal behind the SR15 that makes it different than the colt is that it was supposed to be able to go 25k rounds without replacing the bolt, extractor, springs etc.
personally, i liked a lot of the features and used the SR15 as my primary AR for several years... however, I've changed my mind on a few things this year. i don't like the flush cups anymore, don't use BUIS at all, want a shorter barrel, and basically don't mind regular maintenance (replacing springs etc when they get worn). and I prefer a different stock and rail and trigger and grip.
so the new rifle i'm building is actually more expensive, but exactly what i want now, but what I wnt and what others want may be completely different as a lot of that is personal preference.
but cost-wise, if you start with a $1100 colt 6920, and add a $200 stock, a $250 trigger, a $250 rail, $100 buis, that puts you at $1900, which means you're getting the dually-extractor, rounded bolt, flush cups, ambi controls, and other unique features for about $100. I don't know why anyone who wants those features would think that isn't competitively priced.