I don't know how Trump proposes to pay for the wall. Perhaps Mexico will pay for it. lol My suggestion is to use the foreign aid budget.
The wall will be used to stop illegal immigration. Will it work? I don't know. But it would be symbolic.
And that's where I think
@Bruce Wayne is correct. Nationalism is on the rise throughout the West. Trump, Brexit etc. increase the momentum and shift the Overton window. The wall will be a physical manifestation of a rejection of globalism and a return to putting ones own nation first. I see this as positive step forward.