You really can't get to Belize from Cancún anymore. AeroCaribe used to fly several times a week, but once they got merged into Click they stopped flying into BZE. None of the tiny local carriers like TropicAir or Maya fly there either.

You can take a bus - they are pretty nice with roomy seats and food service - for something like $30 if I remember right. It's a long trip though...count on about 6 or 7 hours, with a change of busses and bus companies in the middle of the jungle.

The other option is to fly to Tikal and then from there back to Belize City on TropicAir. All of these flights are going to be on puddle-jumpers. Belize is absolutely worth it!