Can't answer on the exact price (or whether this Belize dollars or US) but do know that the difference all relates to airport fees. International, of course, has higher and more fees applicable. Another thing to take into considerationi is the time, money and extra baggage handling (by you) required to get from International (assuming you are arriving there) to Municipal.
Enjoy your flight, whichever one you take. The flight is worth it in the end..