All the money can stay in the U.S. The workers will spend the money they make thus stimulating their local economies. Some of the money will come back in Federal and State income taxes. Some money will come back through payroll taxes. If it weren't a political issue, Democrats would call it an investment and a stimulus plan if Obama wanted to build the wall.
The bottom line is Democrats try to focus everyone's attention on the wasteful spending with the hope that U.S. citizens are stupid and don't understand where the money will go. This is how they roll.