It's a 200 dollar tax stamp to make it a AOW. Which is what you are making, when you put a forward grip on a pistol
Honestly, you would be better off making it a SBR (still a 200 dollar tax stamp) and have a buttstock put on it. then you would be free to shoot it with it on or off. With a AOW, you can't put a stock on it.
The forms you will need are 2ea of, form 1, (5320.1), F 5330.20, and FD-258-LE
all of which you can order from the ATF through their website, they come in a less then a week