Congratulations, Diane, on your impending wedding! How very exciting!
Taxis to the airport typically cost $50-$60 one-way and the downtown Orlando area is not as far away as the airport so I would estimate that a cab ride could easily run $30-40 each way to the city clerk's office. Have you considered renting a car instead? Rental cars are available from Disney property. That might actually save you money if you feel you would be comfortable driving on the right side of the road. Our current Moms Panelist from England, Emma, has done this during her Disney vacations. Perhaps you would want to send her a question on the panel to ask her, specifically, how that worked out for her.
Then, there's also the option of
getting married right at Walt Disney World. You could speak to the Disney Weddings Team by calling (321) 939-4610. They have a wide range of options regarding location and price throughout the Walt Disney World Resort.
Whatever you decide, please accept the best wishes from the Moms Panel for a long and happy marriage!
Jill