I rather like this answer on Quora from a Mexican's (Raul Aguirre) perspective, many points you guys are already talking about of course, but this seems like a good explanation from 'down under' on the folly of the wall:
https://www.quora.com/Why-are-peopl...der-wall-between-the-United-States-and-Mexico
"Hello, I'm Mexican living in Mexico new to Quora, my English is not the best but I saw this question online and I'm going to try and give my point of view from this side, I think the US has all the right to secure it's borders and respond to its citizens concerns but building a wall it's useless and a major waste of money and resources,why? Because the majority of illegal Mexicans in the United States go there on a tourist visa and overstay the limit time, a wall would not stop that and if the concern is the smuggling of drugs you have no idea how smart and creative the cartels are the methods they use goes beyond what a wall can do, we are talking about tunnels, drones, direct smuggling through the checkpoints, complicity with US authorities, the sea, etc
Also the net immigration from Mexicans to the USA has been zero from some years now, what some Americans fail to realize is that Mexico is a HUGE country, we are about 130 million people, of wich the majority of us in spite of our corrupt government and the violence in some states are living very happy here and don't intend of going anywhere, If your government REALLY wanted to do something about this issue they should use that money to
-work with Mexico in the way smaller southern border with Guatemala, central america is where they should focus on
-make better immigration laws, cheap labor is something your country will always need, it's a big part of your success, just try to have it in an organized way
-increase security and vigilance in strategic points in the already existing border
-invest in the HUGE problem of your society's addiction to drugs, make programs to help your people and stop the demand of drugs and also stop the flow of illegal weapons that are being sent to Mexico that go to the cartels
This will not only help your country but it will help us a lot as well, Mexico is not your enemy is your ally, we have historic, social and economic ties and is not like we're going anywhere, we should encourage our governments to work together and really put attention to priorities
I don't want to offend anyone but if you really think a wall will change something you're truly oblivious and ignorant to reality and the world around you, your president knows this and is taking advantage for his own interests he's not as dumb as he appears to be, don't fall for it."