That is a NEA CCS (Compact Carbine Stock) PDW Stock. It's basically a modern version of the Colt M231 port firing weapon stock (I still have a couple of those). Google search images for "AAC Honey Badger" and "HK 416C".
These are made in Canada and are not imported all that often, hence very pricy. Canadian residents can purchase for $399. I bought one from Engage Armament a couple of months ago for $425. They had quite a few listed on GB. Part of the hefty price is due to the special bolt carrier that comes with the stock kit.
BravoTac has one listed on GB right now for $499.
http://www.gunbroker.com/Auction/Vie...Item=448931347