According to a leaked Department of Homeland Security report, completing Donald Trump's wall along the US-Mexico border will cost $21.6 billion. Give or take. That's about twice that of Trump's own estimate.
Because money is kind of hard to imagine at the scale of governmental budgets, I did some quick math that might put things into perspective. For $21.6 billion dollars, you could hire about 65,000 border patrol agents at $40,000 per year for 10 years. Add in the 20,000 agents already patrolling the border and staffing checkpoints, and we have 85,000 agents in all to span a roughly 2,000 mile border. Subtract the 700 or so miles that are currently already fenced, and the open range needing to be patrolled is down to 1,300 miles, not accounting for natural barriers.
So, if we then divide that 85,000 three ways (optimistically, sure) to account for shifts and days off and whatnot, we wind up with 28,000 agents guarding 1,300 miles. Lob off a couple hundred miles from that length for pre-existing barriers like the Rio Grande River, you wind up with three or four agents allocated for each individual mile across the entire border. Throw in some lifeguard chairs and we wind up with a border that's under constant view of a live, present human at any given time.
I'm obviously taking some liberties here with a lot of unknowns. But given that Trump wants to spend billions in taxpayer money to solve an imaginary problem, it's kind of relative.
from For $21.6 Billion You Just Could Pay a Line of Dudes To Stand on the Border