When I went on my cruise in 2007, it was $24 flat rate per cab (from Miami Airport to Port of Miami), plus something like a $2 toll. Again, I don't know the current rates.
2007 - Majesty of the Seas (Bahamas)
2009 - Enchantment of the Seas (Western Caribbean)
2010 - Freedom of the Seas (Western Caribbean)
2013 - Golden Princess (Alaska, roundtrip from Seattle)