Bad Math Props Up Trump's Border Wall

Konstantin Kakaes wrote for MIT Technology Review about the cost of Donald Trump's proposed border wall:

Set aside the questions of whether it’s wise to put a wall along the U.S.-Mexico border or who should pay for it. It simply can’t be done at the price Donald Trump has claimed.
While promising to put up a high concrete wall along the Mexican border, Donald Trump said it would cost between $8 billion and $12 billion. Fat chance.
That becomes apparent after you look at what’s already on the border. After initially proposing to wall off all 2,000 miles, Trump said the wall could run along roughly half of the border, with mountains and other natural barriers blocking immigrants from crossing elsewhere. And on the portion where Trump envisions a wall, there are already 653 miles of fencing—some designed to stop cars, some to stop pedestrians, depending on the likeliest mode of crossing in each section. Building those fences has cost $2.3 billion since 2006.