http://www.cnn.com/2017/01/23/politics/border-series-texas/index.html
The US-Mexico border spans 1,933 miles. It runs from the Pacific Ocean to the Gulf of Mexico, across California, Arizona, New Mexico and Texas.
In short, if you do any thinking about it whatsoever you will realize it’s a completely impractical and unworkable idea. That we might actually spend $10-$25 billion on this is nuts.