Cancun, Playa del Carmen, and Tulum are not "cheap" Mexico.
These are the most dynamic and expensive regions of the country, where prices and demand are driven by investors from the US, Canada, and Europe. The Caribbean Sea, the uniqueness, and the natural beauty of this region cannot be cheap.
However, many residents of large cities with high prices and taxes (New York, Los Angeles, Toronto, Moscow) rate local prices, not in the hotel parts of resorts, as low.