Wall paid for - by the American citizens, not Mexico. Tariffs get tacked onto the sales price.
I keep getting tripped up on that too. THe best way to make Mexico pay is to simply create a better climate in the US for business, thus removing the incentive to move work there and in fact, creating one for moving it back to the USA. Then the wall becomes even more important because the initial reason illegal immigration started was because there were better opportunities here, so typically a man would sneak across the border and work illegally and send money back home to his family in Mexico.