I've seen 308 lowers at gun shows for $250-300 range. A 308 is not an AR-15, but a different animal. 308's have different lowers, Rock River lowers accept FAL magazines. DPMS uses the MagPul mags. Most are now standardizing on the MagPul especially since Remington bought out DPMS and Bushmaster. You may need to go to a gun store or gun show with your upper to find a lower to fit it.