Flying out of municipal generally saves you money, even with the US$20 cab fare, if there are three traveling together and certainly it's worth it if there are four, as with four you save a total of about US$75 each way to San Pedro (or Caulker) even after the taxi. Flying to other destinations in Belize the savings are less at least in percentage terms.
When I fly out of municipal, I like to fly Maya Island, because Maya has a nice new air-conditioned and spacious terminal at the municipal airport, and also a new building at Placencia.
--Lan Sluder