If you fly from the International Airport as Elbert suggests there'll be no need for a taxi. At the Caye Caulker end there aren't any taxis, just bicycle carts.
You could fly from the Municipal Airstrip for around half the fare charged from the International. I believe the taxi to get there is somewhere above US$20, but no doubt someone will correct me on that. Bear in mind the time and effort involved, and the uncertainty of getting on any particular flight. If you fly Tropic from the International and book it beforehand via their website they will get you to CC regardless of international flight delays. That is what I'd do.