I gave my first born child for the Brobee 870 to AR15 stock adapter.
The AR15 stock is a rock river arms 6 pos ($125), and the AR pistol grip is a Fobus/First Samco G27 grip ($35). The front pistol grip ($35), 6rd side saddle ($39), mag extention ($65) and Barrel Shroud/Heat Shield ($30) are TacStar.
The flashlight was bought off SubZero on gunnutz (Nuwai Q3- 3 Watt Luxeon LED Flashlight $45), and mounts with some shaved 1" rings to the left side of the barrel/mag extention clamp. The laser was custom manufactured by
www.z-bolt.com and is a 22mw green unit with a custom mount to the barrel/mag clamp ($135 for the laser, $50 for the mount).
The 870 top rail was installed by Williams Arms. Cost $60 including drill/tap and install if I remember correctly. The shortie 12.5" barrel from Dlask was $299, the 32" super long barrel I picked up for $50 at a gun show (unfired)...