I am doing this in a few weeks, but going to San Pedro.
Flying into Cancun and taking the bus from the airport to Playa del Carmen. Spending the night there. Then taking bus from PdC to Chetumal, then water taxi to SP. (I am presuming the that water taxi from Chet makes stops at CC)
I am estimating that the 2 buses will cost me about $30 USD, then night in PdC about $50 USD, water taxi about $40 USD. So about $250 R/T to travel thru the Yucatan.
My R/T airfare from Chi was $332. Yes, I burn a day each way traveling thru the Yucatan, but it is still cheaper than flying into Belize City. I could not find airfare any cheaper than $750-$800 USD and that only got me to BC.