If what you are trying to do is actually stop terrorists and drugs from getting in.
1) No terrorists have ever been caught at the southern border. The 3000+ person number bandied about (and utterly unexplained, even this afternoon) by DHS is meant to be misleading.
How do I know? Because if any ISIS or Al Q people had ever been caught at the border, it would have leaked immediately and been splashed on every news report for years. Trump would declassify it right away.
The 3000 are gang types. 100% of them.
2) The bulk of the drugs coming into this country come in cars or trucks. A wall would do nothing - nothing! - to stop the flow of drugs into this country.
also!
Immigration has been net zero for years. Migrants move back and forth, based on seasonal work patterns. A wall would actually keep them IN, since it would be harder to move back and forth. You'd end up with MORE MEXICANS in the country (gasp!).
So there would be zero benefit for the cost of five billion and - much more - a terrible, unnecessary message sent to our southern neighbor.