Go Airport Shuttle (Florida Limo) - a Limo for 2 for $85.00 total as stylish way to end your cruise. http://floridalimo.com/. Quality Limo are $12 pp plus gratuity for 8 people. Finally do a one way car hire from Alamo in FLL and drop off at Port of Miami (location is the same for airport or cruiseport).
Miami airport is one of the worst international airports with access to shops and food. So make sure you take something to eat and grink with you. Taxis are fair value to the airport and probably your best be.