If Trump builds a barrier/fence/wall and it funnels people to ports of entry, then many many Mexicans will not make it into the country that do now by sneaking across the border...thereby saving billions of dollars in welfare, food stamps, health care costs, housing, education, etc. THAT's how Mexico will pay for the wall.
That welfare trope is transparently false; non-citizens are not eligible to receive those benefits.
There is NO evidence to suggest fraud on that scale.
There is no sense in which Mexico will pay for the wall. El Talon de acero will never be built.