In order to get to Cancun, you'd have to do the following:
1. Take a cab from the Cozumel airport to the ferry terminal (about $14, if I remember correctly)
2. Take the ferry from Cozumel to Playa del Carmen (there is NO ferry from CZM to CUN). For rates, check
http://www.granpuerto.com.mx/en/
3. Take ground transportation from Playa del Carmen to Cancun. A taxi will probably run around $50. If you're adventurous, you can take a collectivo (bus) for (I'm guessing) around $4-5 per person??
It's going to take 15 minutes to get to the ferry station. You'll want to allow some time to purchase your ticket, etc. Probably 30 minutes. The ferry is about 45 minutes. A taxi will take about an hour to get to Cancun. So it's probably close to three hours in total.
The upside is that you get to see a lot more of the Yucatan than if you just flew directly into Cancun. You probably won't save a ton of money, however.