On this site go to the "travel hints" and then scroll down to Getting there - from Mexico. It tells where and what. Takes all day from Cancun/Playa del Carmen and cost some where under $100 US each way depending on if you fly or take the boat. I have not done it yet but plan to next winter. My advice is if you are staying 10 days or more it maybe worth the time to save the extra money. How soon are you planning to come down? Right now Belize has ban Canadians from entering the country.
I do not know for how long. There is an article in the San Pedro newspaper, which you can read on this site.