The answer is in his dreams! He thinks that he will build the wall between the US and Mexico border but Congress has not given him any money other than maybe enough to build a few prototypes. Hopefully, after the US midterm elections in November 2018, there will be even fewer Republicans in Congress and it will be even harder for Trump to get the money.
Trump’s 2019 budget request asked Congress for $1.6 billion to build part of the wall, mostly in Texas. Keeping this in mind, Presidents’ budget requests sometimes look nothing like what Congress ultimately agrees to give. How could anyone agree to spend that much money on estimates that come to between $17 million and $25 million per mile? Keep dreaming Donald!