A pure hourly rate would seem easy to figure, but I dislike that there is no financial incentive for the negotiator to actually get results. The ideal situation would be for the negotiator to also benefit financially from the result, as he/she would likely be more inclined to push a bit harder.

Perhaps a combination of an hourly rate and either (a) % of salary - which would be very small I assume or (b) % of difference between initial offer and final offer - which would be larger than the percentage in a.

I'd have to think about it, but in the short-term I'd charge my standard hourly coach/consult rate to job seekers which is currently $60/hr.