And now all goods made in Mexico will be slapped with huge tarriffs and import taxes (this means that Mexico will pay for it). The Mexican President will either step down from his posturing (as he's very unpopular in Mexico) or fall on his sword on this.
It actually means USA people will pay for it, twice, because they will pay for the actual building of the wall, and now for overpriced goods.
People there will buy less stuff from Mexico, so it'll also hurt people that choose not to pay for it and will have to find more expensive alternatives.
USA only loses with that.
Mexico loses as well, but they can sell their goods for other countries instead of letting USA try to rule them.
Hmm, you realize that the proposed tariff or import taxes on Mexican goods (based off last year's exports alone) would pay for the estimated cost of two walls across the entire border? Estimated costs are $15-25 billion and the proposed tax is 22% with $236 billion exported into the US in 2015.
And those "expensive" alternatives are either from the US or from other Central and South American countries. When one of your largest trade partners has a fairly substantive trade deficit in their favor, it's not uncommon for politicians to try to balance it out. When you have the leverage in trade situations, you don't concede. :-/
Edit: Ofc, we could truly hard-nosed and eliminate all foreign aid to Mexico and use that to pay for it. The United States gave away $419m in foreign aid to Mexico in 2013.