Let us assume that Mexico never contributes a nickel. Illegal aliens cost Americans an estimated $113 billion per year. Each year an estimated 700,000 illegals gain entry and stay in the US. That is an increase in the number of illegal residents of 5 percent per year.
A five percent increase in our illegal alien population would be an additional $6B above our current expense.
The wall is intended to keep that number from continually escalating.
In year 1 of a wall we will save $6B. In year 2 of the wall we will keep out another 5 percent so we will have saved an additional $12B for a total savings of $18B. By year 3 we save an additional $18B for a total savings of $36B and in year 4 we will save an additional $24B for a total savings of $60B.
If that wall could be built for $21B and put up in one day, by the time our president takes the oath in 2020, the savings of NOT having to pay for those new border skippers will be nearly three times the the expense of the wall.
Only a fool believes that the wall is costing us money. If the wall stands for only ten years, it will pay for itself ten times over.