Last weekend I took supershuttle to port from airport it was $9.00 one way . The Royal Carribbean bus shuttle told me I could go with them for $11.00 a taxi said it was about $18.00 . Being as I was alone supershuttle was my best deal if there are 2 or 3 of you a Taxi would be cheaper . Also when I came off ship supershuttle was right there $9.00 back to airport waited only 5 mins very easy .
The taxi is far and away the fastest and most economical FROM MIA to the Port of Miami. They charge a flat rate, regardless of the # of passengers and there is no waiting for a bus to fill up, and so forth. Down side? It may take a few extra minutes to get a cab on the way back to MIA from the Port... although there are plenty of taxi's.