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"Should" is a moot point. There is zero chance that Mexico will pay even one peso for a wall.

But, what if it was a Great Wall? Like in China. Tourism could cover the cost.

While there will probably be no ceremony where Mexico's president is writing a check and cuts a ribbon or something like that, the US could raise money from tariffs. They are many possibilities to get the money from Mexico.

Mexico could build a wall on their southern border, learn English, and become a state, or 3.