From Cancun City, buses depart approximately every two hours. You can find schedules & fares on the ADO website...
http://www.ado.com.mx/wadod/compra.jspIt's in Spanish, but another site called ticketbus has an English language option...
www.ticketbus.com.mx/wtbkd/I18n.tb�
Cancun to Chetumal is about 300 pesos on an ADO express bus. The water taxi from Chetumal to San Pedro is about US$30. The ADO overnight express bus from Cancun City to Belize City is 496 pesos.