What does it cost us to have 1 illegal in the country stealing from us through welfare, tax cheats, additional law enforcement, theft, crime, prisons, etc? Some more simple math;  $5.2M / mile of material cost is $1K/ft. Assuming a high installed cost of $20K/ft is $105M/mile of barrier. If the cost of an illegal is only $10K/year, a million illegals costs $10B/year.  That equates to ~95 miles of barrier per year at $105M/mile.  Easy to see how quickly the wall pays for itself.