Taxis are one of the mainstays of getting around on Ambergris Caye. It costs US $5 to get you from San Pedro (the main town on Ambergris Caye) to 5 minutes in either direction, or about a mile in either direction. The prices will go from $5 to $25 to get you 3 miles. You can find taxis anywhere at the airports. They are a relatively common form of transportation for everyone on the island.
If you want to go to some of the resorts, $25 would take you all the way up to about 3 miles north of the tip of the island, and there is also a toll fee to get across the island.
Taxis in Belize do not have meters, so you have to negotiate a deal with the driver first, but for the most part, I have never had the problem of someone trying to overcharge me. Personally, I have a consistent taxi driver, so once I am here, getting a taxi or negotiating fees is not a problem for me. Generally, just check with the driver ahead of time and ask how much it is to go to where you are going. They will typically tell you, but definitely ask first before you get to your destination.
(Arial view of the roads out of the airport in San Pedro, Ambergris Caye, Belize, pictured.)