40-50 bucks. The ACS also has storage you can access WITHOUT having to remove the stock. Seriously, try accessing spare batteries in a rush with the SOPMOD. The ACS also has storage in the bottom potion of the stock, and a tension lock like the CTR.
It also has a strike plate at the bottom, which is nice for real rifles (in a class I took with the local swat team we had to break a car window with the butt of the stock).