I have flown a few times from Chetumal to Mexico City. They had 2 flights per day, and quite a few people going to London, and other destinations in Europe used to take that route. When you use Cancun you might have to stay overnight, better stay in Playa, so you are one hour closer to Belize. The ADO bus takes about 4.5 hours and costs about US$ 20, take the ejecutivo. By taxi it can cost between US$250-350, and takes one hour less.