The point is that the 556 is not really that expensive. It's easy to say that a 556 is about "bling" when you don't know much about the gun itself. Those who know about the Sig 556 know that there are a few advantages over the AR15.
One of the biggest differences is that unlike the direct gas system in the AR15, the 556 vents it's unburned powder and gasses out of the front of the weapon instead of clouding up the action. That means it is easier to clean, more efficient, and more reliable.
That is just one of a few of the things that makes it a better rifle. There's also the fact that it's chambered in 5.56 which makes it even cooler in my book.
BTW, if the zombies attack, they be on the receiving end of an AK or Saiga 12.
If you think venting the gases out the front instead of using them to ensure the next round is chambered properly and that it is "5.56" makes it worth double what a fully functional, reliable and accurate AR-15 is gonna cost, then by all means, get a dozen. You can get ARs chambered in 5.56 too.... or 6.8 rem.... or 7.62X39 or dang near anything else you want.
I'm going to get some 6.8 rem SPC magazines and a 6.8 upper for mine. If the zombies come they will be eating 6.8, but I can shoot 223 for practice until that happens
And if the zombies are bigger or further away, I have something for that too.
Seriously for the price of a SIG 556, a guy could have an AR in 5.56/.223 and a full 6.8 rem spc upper to go along with it and a pile of ammo..... and probably a few other accesories they might want.
And as for that powder and gas clouding the action.... I've shot over 200 rounds at one sitting through an AR with absolutely no problems. I'm sure I could shoot 1000 at a time between cleanings if I wanted, but my pockets aren't that deep.