It depends on how you enter the country, how many people in your group and what time you are arriving to recommend the best ride to Caye Caulker.
To fly from the international airport its approx 94bzd each way to Caye Caulker, and a 15 minute spectacular ride.
From the municipal airport its 54 bzd each way, but add on the cab ride from the international airport, which is about 35-40 bzd for the whole cab (not per person) and a 15 minute ride.
The water taxi is 25 bzd round trip, but add into the cost the cab ride (approx 40 bzd each way) te trip takes about 45 minutes by water. The last boat leaves Belize City from the Marine Terminal at 5:30
If you are traveling with even a small group
its more economical to use the water taxi.
If you use the search feature in this forum, you'll read quite a bit of discussion on this topic.