Hi Mazinya, Depending, if you have more people in you group, a car rental might have been cheaper, but certainly a lot more hassle, if you ask me. I agree the shuttle would be a comparable price for only two, and easier. Certainly the shuttles will take you directly to the port.
There are several companies out there to choose from. A web search will find them. Just type in "FLL to port of Miami shuttle". Several came up. Looks like prices run about $30 per person each way. The ones that have you share the ride seem cheaper.
Some companies charge per trip, anywhere from $50 to $75. for 1 to 4 people, which I think is quite similiar to what you'd have paid to rent a car.
(I can't recommend a specific company as we've not used one between Fll and Mia. Every time I've researched it, the extra cost and trouble of the transfer, seemed to more than eliminate the savings of flying to the the other aiport, and so we always end up booking the nearer airport even if costs more.)
Good luck.