Cruises from Cancun to Isla Mujeres?

Some of them are cheaper, some of them are very expensive because of the services they offer. A trip to Isla Mujeres starts at 27.50$ and includes snorkelling, bar and shopping, or you can choose the version with meal included (35$).
If you want an all-inclusive day cruise to the island you have to pay as much as 44$ (go to M/v Aqua II).

Pesonally, I didn't think it's worth the money so I chose the cheapest version (without meal) for 27.50$.