Back in December 2009 we flew TimAir to Negril. There were a couple of cabs in the parking lot and we hired one to take us to CSA. I think we paid US$10 for the five minute ride. We probably paid too much but it wasn't a big deal. Fix a price before getting into the cab and make sure that the cab has red license plates. These are the insured drivers.
TimAir will call CSA for you and then they will send a van. This may take 20-30 minutes. The first time we did it this way and waited on the van. The second time we just hopped in to one of the waiting cabs. It was $10, no waiting and we were at CSA in 10 minutes.
On your way home CSA will arrange the van to take you to the Negril air strip. We tip the drive $5 for the free trip.