The taxi fare is $24, plus $2 (I'm assuming for the toll that you have to pay to get to the port from Miami). I just got back from my cruise on the Majesty, and I can guarantee you that you're going to love it!
Also, just tell the cab driver that you want to go to The Port of Miami to the Majesty of the Seas. Royal Caribbean uses the Port of Miami.
2007 - Majesty of the Seas
2009 - Enchantment of the Seas
2010 - Freedom of the Seas