This has been on my mind since last year, so I thought I'd air it. There are lots of discussions on the board about the cheapest ways to travel between MBJ and Negril, and often a comment when somebody says a taxi ride is $70 to the effect that that's too much.
We always have a pre-arranged pickup with a driver we know (red plate cab). We pay him what we consider to be a fair amount, which is in line with Rocky's price (to name one example familiar to board members) with an adjustment because we have our driver bring his van, and he waits for us to clear Immigration and Customs and then makes stops to pick stuff up so we're using up his day.
Last year, I observed tourists playing off the taxi drivers against each other to get the cheapest rate. They were looking for $40. one-way for multiple people. They didn't get that, but they got under $50.
Taxi rates in the US run around $2./mile on average - some places as low as $1.60, many places in the neighborhood of $2.50/mile, and often a premium is charged for long distances. MBJ is 50 miles from Negril (somewhat further from the cliffs). My house in central NY is 52 miles from the Syracuse airport, and the published one-way taxi fare from my town to SYR is $145. It is certainly not cheaper to operate a vehicle in Jamaica than in the US; fuel costs are significantly higher, and the roads put more wear-and-tear on the vehicle. Negotiating a fixed cost doesn't save you much money in the US - my experience is that a driver might offer you a flat $50. rate for a ride that would be $60. on the meter, but that's about it. And just as in Jamaica, cab drivers expect a tip above the mileage cost.
I am sure that if you keep trying, you can (as in the case I refer to) find a driver desperate enough for cash that he'll take you at a loss. You are negotiating from the position of power, after all. But I suggest that you have mercy and pay a rate that fairly compensates the driver both for the vehicle and his time.
Note that I am not in any way against using the advertised rate for a service; if there is a bus that advertises an $18 fare, fine. I'm talking about driving a hard bargain to get a lower price by playing people off against each other.