It's a set price from MIA to the port. Used to be $18 PER TAXI, not per person. Think it may have gone up a couple of bucks, but I can't recall, as we've picked up rental cars the last couple of trips down there.
we are flying in the night before and staying at the miami days in they pick you up from the airport and drive you to the port the next day. don't have to worry about late flights if you are flying in the day of the cruise, our honeymoon was 5 hours late!!!!!!!!!
When we flew into Miami, it was $20 plus tip. It was cheaper than the charter from Carnival. Had no problems with the driver or the price. Like you said, it is posted. We also rented a car, since we flew in the day before, (always like to arrive early), we rented thru priceline with a great rate of $19.00 and then returned it to the airport and took taxi to port. Worked out great.
There is a set price that the taxis must abide by from the port to the airport in Miami, regardless of how many passengers or how long it takes: $21. The taxi will have a small sign beside the drivers picture that states this price (which includes tolls).