My son had this exact thing done on his M3, except the cutout is just before the mufflers. It cost him $500 total including the valves and switch, including labor to install and weld the valves in place. He did the wiring himself. Sound is awesome open. Better than my $7000 aftermarket system on my M3.
That being said, it does not create any horsepower like my system does. My full race exhaust system gives me about 35 HP. We dyno'd our cars with valves open and closed, and there was 2 HP difference on his car.
2010 M3 Coupe, 2010 X5 35D
2011 458 Italia, 2013 M5