In Mexico (and other countries outside the US) resorts, hotels, public transportation operators, pharmacies, and excursion groups do indeed give lower rates to Nationals, and to Permanent Residents (expats)...
Years ago we used Funjet and Liberty Travel but none recently. Try Googling Cancun TA’s...

Cancun hotel zone is home to resorts with price points from very exclusive resorts to nice condo rentals to timeshare rentals, the majority oceanfront. Do you want an AI? Or would a less expensive condo be a nice option?

[EDIT] Also, have you looked at the Riviera Maya to the south of Cancun? Several beautiful resorts line the ocean from Cancun to Tulum...