LOLMexico will pay for the wall.
It's unlikely it will be in an official capacity but the savings gained in a reduced drug and illegal immigration flow of money out of the US and into Mexico will more than make up the difference.
the Easter Bunny and Santa Claus will kick in some money as well I am sure.
We/you may pay some for the wall in higher food costs from tariffs on imported foods and autos, etc.