The money from Mexico will be have to be taken from some other place. Let's say they cut their drug enforcement program to pay for this wall. It leads to more drugs entering the United States ,that leads to the federal government hiring more DEA agents ,It leads to big city's having to increase their drug enforcement police. It also causes things like murder , theft ,and other crimes to go on the rise. Where are we saving the costs ? Nowhere we're just paying in a worse way.
Ok they don't take money from drug enforcement they take it from social programs. That creates more people wanting to flee to the United States. So you think a wall is going to stop people , it didn't stop the Mongol horde from attacking the Chinese. Their wall was massive. For a wall to work it can't be a thin cement war it would have to be thick enough to withstand explosives ,high enough for ladders not to work ,and it would have to be buried deep enough not to be tunneled under.
It would have to be patrolled 24-7 by lots of men , Use Drones some people say might work until some smart hacker learns to hack them and take control. Cameras right nope a camera can be beat by a simple bullet or a can of spray paint.
The cost of this wall keeps going up as the benefits keep going down. In the end no wall will ever defeat a person with nothing to lose whose desperate.