First off, it's painfully obvious that you're trying to shadow-sell this gun for $1500 and disguising your ad as just an attempt to gauge value.
Second, for some reason, you've decided to spam your "ad" on multiple topics here. Few people will send funds for a $1100 rifle to someone who has TWO posts, both of them trying to deceptively sell a rifle. So good luck with that.
But still, to address your feigned question, unless your buyer is in a state where the lack of a threaded barrel and brake is a requirement, that's going to be a net decrease on the value, of course. Hazarding a guess, the overall value is probably about 15% less than the same configuration in a non SCM version - but again, it depends on the buyer's state law.
Overall, as-is, without sights, in that config, you could probably get about $1100 for it if you're not in a hurry to sell it, and perhaps $1000 if you want to sell it faster. A buyer in a state where that's the only legal way to get a SIG 556 probably wouldn't balk at paying $1100-1200 though, as they don't come up for sale often.
It's still an OK time to sell guns, but we seem to be gradually moving towards a buyer's market due to the economy.