I threw out a few price points when I was chatting with folks, and across the board, everyone was pretty surprised that I'd be charging them. In order for this to be worth my time, I'd need at least $5000 or so per head, and that seemed like an insurmountable figure for people. The best I could get was something along the lines of, "If you can guarantee that I will make $20K more per year, then I can give you a cut," but I didn't really like that either because it still creates the wrong incentives, essentially making it not in my interest to ever refer anyone into a cash-poor, equity-heavy situation, or really into any company that couldn't throw out a lot of cash.
The closest thing I've seen to what I described is offerletter.io (also mentioned on this thread), but I think that has a good chance of working out because of the focus on just the negotiation aspect.