It's very inexpensive and easy to do a one way rental in this area. The key is to return the car to a location very close to the cruise port in Miami. Enterprise has a location at the Radisson on Biscayne Blvd. and I think Alamo has a location very close to the cruise port. Dollar rent a car is usually the cheapest but then you have to get from the airport to the cruise port in Miami.
We're flying into FLL and taking a rental car and going down to the Keys for a couple of days then the night before we cruise we're returning the car at the Radisson (so we don't have to pay for parking and the rental for another day) and spending the night at the Radisson in Miami ($45 on priceline) so we can relax a bit in the morning and not feel stressed or rushed. Then after a leisurely breakfast, we'll take a quick, cheap cab ride to the port about 10:30 a.m. or 11:00 and have a great trip.