ADO is probably the best & safest bus line. They operate express buses between Cancun & Chetumal, and they have an airport shuttle bus between the airport & Playa del Carmen.
But there are other bus lines -- Mayab is one of them -- and there are non-express and second class buses that will be cheaper than the ADO express buses.
Schedules & fares can change anytime; check for updates at:http://www.ticketbus.com.mx/wtbkd/I18n.tb?language=en®ion=UShttp://www.ado.com.mx/ado/index.jsp
Note: On the Mexican bus websites, just the dollar sign is used to represent pesos.
On the way down, you might want to stop at Playa or Tulum or Bacalar. Maybe even a night in Chetumal.
From Chetumal, there are two afternoon water taxis to Ambergris.
A cheaper but longer option would be to get a Belizean bus to Belize City, then get the water taxi.
The new route between Cancun and Belize City bypasses Chetumal, and it might be more expensive, but the water taxi from Belize City to the cayes is cheaper than the water taxi between Chetumal & Ambergris. You can decide which route fits your budget.
You'll find all the trip durations and fares on those websites.
Also check this site...http://belizebus.wordpress.com/