Some of the Gunbroker asking prices are ridiculous anyways. And, I believe that some of the final sales prices don't really reflect the value of the gun. Instead, they look more like a bidding war where two people couldn't let go even when the price got too high.
I got my 556R SWAT recently for about $2250. But, it also came with a Browe BCO; so, that puts the gun's price somewhere between $1200-1600 and the Browe price somewhere between $600-1000. Not so bad in that light.
You can find good deals on gunjoker. You just have to be patient, persistent, sift through the garbage a bit, and be ready to make a move asap. Remember, fairly priced 'buy it now' options go extremely quick. So you might not even be seeing the rifles that are posted up by reasonable folks. I just picked up another 556 last week. Price would have been good four years ago. Sold another of mine at a very far price to pay it off the next day. While I agree with you; there are certainly a lot of elevated prices on GB. I also don't see those higher priced Sig 556 rifles selling. At least not without remorse on the buyers part (as Philipat suggested).