In addition to the information Wendy gave you, I'd note that you can also fly from the municipal airport, for about one-half the fare from international. Of course, if you're arriving at international you have to take a taxi to the municipal airport, which costs US$15 (for up to four or five people), so it's a real savings only if you're not traveling alone.
Tropic Air (tel. in Belize 011-501-24/5671 or 800/422--3435 in the U.S., //www.ambergriscaye.com/tropicair/index.html or www.tropicair.com)
and Maya Island Air (tel. 501-23/1140, //www.AmbergrisCaye.com/islandair/index.html or www.mayaairways.com)
both fly to Ambergris. Some people prefer Tropic, as it's a bit larger; others prefer Maya Island, saying it's a bit friendlier. I think both are about the same. Round-trip fares for the 20-minute flight are about US$42 (municipal) and US$84 (international). Flights leave all day long. They also stop at Caye Caulker, by request.
Belize First Magazine http://www.turq.com/belizefirst/