Belize City to Cancun: BZ$55.00
Belize City to Merida: BZ$79.00
Cancun to BC: MX$474 (MX$440 from Playa; MX$382 from Tulum).

Crossing from Mexico to Belize
When you leave Mexico by bus, the bus will stop at a small immigration booth just before crossing the bridge to Belize. Passengers get off, without luggage, and present their passports with FMM (Mexican tourist card/form) and receipt to the immigration officer. Whenever I have crossed by bus, he has always asked (in Spanish) for MX$100 (thatís pesos) or US$10. I have heard that since January 2010, he has been asking for US$20 from all bus passengers.

Beware! This is a scam. Your passport might get stamped, but there is no departure fee if the FMM fee has already been paid. If the FMM fee has not been paid, you will need to pay that fee (MX$262).

Passengers re-board the bus, proceed across the bridge over the Hondo River and stop at Belize Customs & Immigration. Passengers (with their luggage this time) go into the immigration building. For citizens of Canada, USA and many other countries, there are no visas or fees required when entering Belize. You will get a stamp in your passport allowing a stay of 30 days. Citizens of some countries require a visa which must be obtained before entering Belize. After completing immigration and customs, re-board the bus.

Crossing from Belize to Mexico
When you leave Belize by bus, the first stop will be at Belize Immigration. Passengers, without luggage, get off the bus, enter the building and present their passports to Belize Immigration. For visitors who have been in Belize for more than 24 hours, the fee is BZ$37.50 (US$18.75), payable in BZ$ or US$.

These sites explain departure fees for Belize:

Passengers re-board the bus, proceed across the bridge over the Hondo River and stop at Mexican Immigration where they get off the bus with their luggage.

Passengers go into the immigration building, get an FMM, complete it and then present the completed form with passport to an immigration officer. After your documents are returned, walk out of the immigration building, walk ahead and go through customs. Re-board the bus. The fee for the FMM (MX$262) must be paid at a bank before you leave Mexico (do not pay at the border). Visitors in transit can pay at Cancun airport.

The whole processs of going through both immigration points can take anywhere from 20 to 50 minutes depending on the number of passengers on the bus. When I have crossed on my own (and there are no long lines), the process takes about 15 minutes.