From a FlyingBlue perspective it's only 12 XPs (6 each way as it's a domestic flight)
In the case of Delta... MEX-CUN would earn 2397 MQMs 639 MQDs 2.14 MQD Ratio while Delta Diamond members would obtain 7031 RDMs, Platinum 5753 RDMs, Gold 5114 RDMs, Silver 4474 RDMs, Base 3196 RDMs