The other option of course is to fly from the municipal, rather than from the international airport. Fares are almost exactly 50% off on flights to San Pedro; for other destinations in the country discounts are mostly 25 to 35%.
You still have to pay the US$15 taxi fare to municipal, but if you are traveling with other people, or arrange to share a taxi (do it inside the airport, not at the door), the per-person taxi cost isn't much, and you save about US$45 on a round-trip fare, which is enough to buy, lessee, about 25 Belikins.
Just FYI, the water taxi fare from Belize City to San Pedro is US$12.50 one-way. There are three Caye Caulker Water Taxi boats a day (at 9, noon and 3) from the Marine Terminal, plus the Triple J at 9 (from Court House wharf), Thunderbolt at 9 and 1 (from the Texaco gas station on N. Front St.) and the Andrea at 3 p.m, from the Court House wharf.
Belize First Magazine http://www.turq.com/belizefirst/