Welcome to Cruise Chat.com
Can I first say your going to love the Mariner, it's an awsome ship.
We did the same thing last year, flew into Orlando a few days before sailing. I searched around the net and found a few good quote for transport and got one for $90 one way, SUV type of thing. Most of the other quotes were around this price too.
Sorry I can rememeber the name of the company I used, but it give you a ball park figure to start with.
If your wanting to be really sad like me check this website, it's a webcam showing the ship every Sunday leaving the port. http://www.portcanaveral.org/multimedia/webcam.htm
It will give the scale of thing as it sits next to one of the Disney ships too.
Have a good one