Walls keep people in as well as out. If Mexico pays in full for a wall, it will want total control of it. Currently, the border maintenance and operation is based on co-functioning of Mexico with the US and is a border “zone” about a hundred and twenty miles deep. A wall paid for by Mexico would be potentially a containment wall for Americans based on policy determined by Mexico alone. One cannot expect that Mexicans would be kept out and Americans not kept in.