There's tons of taxi guys at the airport but you'll have a hard time bargaining a private taxi down to $25. If you can find a taxi taking a group to Negril you can probably get on for $20 or so. Route taxi (fixed route group taxi, like a bus in the US) from MoBay to Negril is just 300J which is like $3 US, but it doesn't take you point-to-point - you'd have to get to the bus station at each end. Not hard to do but you probably don't want to hassle with that on your first trip.

Hey, enjoy Jamaica!!!