Per Shikha Dalmia at Reason:Just a single-layer fence -- not a wall -- on the 1,300 miles of the open Southern border will cost upwards of $6 billion -- assuming, as per a CBO study, pedestrian fencing costs of $6.5 million per mile and vehicle fencing costs of $1.7 million per mile. A single Border Patrol agent costs about $171,400 annually. So tripling that force would add up to a whopping $7 billion or so more a year, according to the CBO. Annual maintenance costs would be hundreds of millions of dollars. In short, the total hit if cost projections don't balloon -- a big if, assuming that Trump won't use illegal Mexican workers and will use only American steel -- would be somewhere close to $15 billion upfront, give or take, of even a modest version of Trump's plan.There's something to be said for the "price tag policy" adopted by Israeli settlers squatters in Palestine. The purpose of that policy…