In Cancun and the Riviera Maya, you won't be stranded for lack of taxi services. Taxis fairly abound and will be typically on station at your hotel, restaurant or other locale where you're enjoying yourself, even for those of us who find themselves in unlikely places at unlikely hours.
In years past, it was common practice to haggle over fares and woe be to those who didn't determine the fare prior to setting off to their destination. Now, however, fares within Cancun are regulated and the set prices will be posted at your hotel or in the cab itself; although it is a good idea to establish with the driver what the price will be for your destination upon entering the taxi and before setting off.
When traveling long distances, fares are generally still subject to negociation and will vary greatly depending on the circumstances but you will find that, typically, the drivers are looking for no more than a fair price for their services.