Chesnut,
I would say to closely observe the road conditions and the way people drive in Jamaica for a couple of days. Then decide if your confidence and driving skills match the challenges of driving a scooter in Negril. If you have any doubt, don't do it. If you do rent a scooter, have fun, but remember to always drive safely.
As for the cost of renting a scooter, I think $25 or $30 US per day would be about right. The longer you rent, the lower the price should be. Alternatively $25 or $30 a day will pay for a few local cab rides.