I have had many different types of vendors all around the world attempt to- often successfully- overcharge me. Taxi drivers in Cozumel being just one of those. It feels crumby and I hate it, but it isn't personal. It is business and the free market approach taxi drivers, tour guides, street food vendors, and even hotel clerks take all around the world is just the way it is. You think you are being ripped off, they think they are maximizing profit. I've been to a country where the wealthier locals pay more than the poorer locals, so the wealthy walk to the market or ride a bike rather than taking their scooter or car.
The free market also means we get to speak with our dollars, which means don't get in a taxi that quotes a price that is too high. While I 100% believe the stories about drivers not having any change to give back, I haven't experienced it. Ask the driver if he has a specific amount of change, if he says, "no," then just tell him you'll go into the store or to your hotel front desk to get some and you'll be right back- invite him to follow you. Either he has to wait or he magically finds some change. Again, the free market at work.