Disney wants $105 each and if you do it yourself it will still cost you around $90 each, with the Xcaret entry fee ($69), ferry tickets ($22 for round trip), and a taxi from Playa del Carmen to the park and back ($9 plus tip for 4 pax). A lot of hassle and uncertainty to save $15 per person, I think. The ship won't be waiting for you if you do it on your own and are late getting back to Cozumel.
Even though the Disney tour says it is 7-7.5 hours long, remember that includes the transportation time. So you can cut 2 hours out which leaves you 5 hours at Xcaret. Is that really worth it? I never recommend doing a mainland tour on your own from Cozumel. Frankly I don't recommend leaving Cozumel at all. There is so much to do there. Other than the cave tubing anything you can do at Xcaret you can do on Cozumel.
Note the current entry fee for Xcaret is now $69 each.