'Real steel' Patriot Ordinance Factory P12X. Retails at $285 and hasn't been knocked off. The lowest priced part they have is an M4 length handguard at $225 and $245.
The Magpul PRS mounts to an A2 stock's receiver extension. That said, it can easily replace any A2 stock or with the purchase of a $30 adaptor, replace any collapsable stock. I did have to dremel the sling mount a bit - its tolerances were off just a hair.
Cheers.