The Mexican government is afraid that Trump might really tax the $29 billion being sent home to their relatives in Mexico.  A ten per cent tax at Western Union, for example,  would be $2.9 billion per year right out of the Mexican economy.   

They only make $24 billion on their oil each year and that requires investment and labor and material costs to produce.  

The Mexican oligarchs in charge of the Mexican government would have trouble keeping their mistresses and their private jets funded  if that happened!  Oh, the inhumanity!