I love the fore grip it's very functional and I feel like it's more accurate then a traditional vertical grip.
I searched all over AR15.com for mine and I got mine BNIB for $380 shipped. There is a used 512 on there now for $315 you could probably get for $300 shipped. There is deals all over you just have to search like crazy and jump on them immediately. Stuff doesn't last long on forums.