Regarding cabbies jacking-up fares in Cozumel, I haven't really seen it. Like another post stated, a guy trying to get an extra buck or two is not worth getting concerned about.
Wages in Cozumel and other places in Mexico are very low and I always try to be generous to drivers, waiters, maids, DM's, boat crew,and street musicians.
One thing I've noticed more than questionable cab fare is the low-tipping and no- tipping dive tourist. They fly in on a jet with thousands of dollars worth of dive gear, stay in hotels and eat in restaurants while on vacation, but are too tight to pull out a few dollars here and there for tips. Rather than bring up the one or two guys that may