If the price difference going to CUN versus CZM is only about $70 USD less (CUN cheaper) I would not do it unless other significant factors are at play. Just the roundtrip bus and ferry will cost around $50 USD. I actually like the bus/ferry option and developed this crude website to document the process.
Cancun Airport to Playa del Carmen/Cozumel by ADO Bus
Cancun Airport to Playa del Carmen/Cozumel by ADO Bus