Helper is right.
Flight from BZE International to Caye Caulker is: $47.00 US. Flight time: 15 minutes!
Flight from TZA Municipal to Caye Caulker is: $26.00 US, PLUS the Taxi Cab to get you there: another $20.00 US.
Why the hassel to save: $1.00 US?
The same flight that leaves International, picks up at the Municipal anyway (or vice versa), why chase it?
My point is always this, if you will be bothered to take a Taxi Cab to the City, head straight for the Water Taxi Terminal, not the Municipal Airstrip. Then it would be worth it.
Taxi Cab to Belize City - $20.00 US plus water taxi to Caye Caulker - $7.50 US. Time frame: 20 Minutes taxi cab ride + 45 Minutes water taxi.