President Donald Trump’s vow to accelerate construction of a “contiguous, physical wall” along the Mexican border is slamming into a Washington reality – who’s going to pay for it and how? U.S. taxpayers will foot the bill, starting with existing money in the Department of Homeland Security account that amounts to a small down payment. Then it’s up to the Republican-led Congress to come up with possibly billions of dollars more, cutting money for other domestic programs to finance the wall.