The least expensive and best (IMHO) way to get from Cancun to Cozumel is to:
(1) Take the bus from the hotel zone to downtown.
(2) Get off across from the downtown bus terminal, where you can either walk across the street to catch a bus to Playa del Carmen or you can stay on the same side of the street and take one of the white shuttle vans there (the latter is my preferred way).
(3) Once you get to Playa, grab either one of the ferries across to Cozumel.
Expect a total travel time of about 1 hour and 45 minutes for all of the above. For the return, reverse the above steps.