have you also done the calculations on what your itinerary would cost if you flew into Cancun and then did the bus/taxi thing?
For those who didn't refer to the summary on my BelizeBus site...
This is how I do the trip from Cancun airport to Belize�
From Cancun airport to Playa del Carmen, there is an approximately hourly ADO Riviera airport shuttle bus (check the ticketbus site for departure times and fares). Tickets are purchased from the ticket seller where the bus is parked. To get to the bus, go out of the airport terminal, turn right and keep going to the other end of the terminal.
The bus trip is about 1 hour and costs MX$106 per passenger (as at October 2010).
The airport shuttle bus terminates in Playa del Carmen at the bus terminal at the corner of Quinta Avenida and Avenida Juarez (5th Avenue and Juarez Avenue). If you want to continue to Chetumal immediately,
get a taxi (MX$20) or walk to the other bus terminal (Terminal Alterna) at Avenida 20 and Calle 12, where the bus departs to Chetumal (the airport shuttle bus might stop at this terminal on the way to the other terminal). Or, stay a night in Playa.
Here's a link to a map of Playa del Carmen showing the location of the two bus terminals:
http://www.travelyucatan.com/playa_del_carmen_mexico_hotel_map-a.phpFrom Playa del Carmen, ADO express buses depart frequently to Chetumal (and there are a few ADO GL and OCC buses for a bit more money).
The trip takes about 4.5 hours and costs MX$214 per passenger for Primera Class (as at October 2010). Check the ticketbus site for up to date schedules and fares.
In Playa or Chetumal, decent hotel rooms can be found for MX$400 and up.
A taxi to the water taxi in Chetumal costs MX$15 to 20 (depending on distance).
The water taxi to San Pedro is about US$30 per passenger (check the websites for updates).
(MX$ is Mexican pesos)
The Cancun to Chetumal flight is a good option depending on the flight schedule. It's sometime unavailable.
http://www.mayair.com.mx/I go this way because I'm never in a hurry, and enjoy the time in Mexico.