You can also take the bus or rent a car in Mexico to the border of Belize. Once in Belize....either take a bus or a cab to Corozol....it's not far....and then fly from Corozal to San Pedro, Ambergris Caye.
The flight from Corozal to San Pedro is $70 US round trip....and leaves 3 times a day....7:30am, 10:30am and 3:30pm. That is probably about a 30 minute flight or less.
You can visit Tropic Air at: http://www.tropicair.com