Assuming the destination for their wedding is of their choice and not because of some economic forces. If you really don't want to go because it is cheaper to visit Mexico for twice the time, I'd recommend either offering to cover her expense or split it with her as a peace offering.
All in all, while a 2wk vacation to Mexico could be cheaper, it isn't about the destination as much as it is about the event. You're there to support your family for an important event. So, I think there is some sense of being an azzhole about not going simply because you could go to Mexico for less. The problem is, the wedding isn't in Mexico, it's in the DR so the trip to Mexico would be all about you and the trip to the DR is about them.
Anyhow, just my thoughts.