They will hire Mexicans to build it for them. But I think they (America) will not build that wall, they have larger issues to solve and allocating budget for fence building is gonna cost them more than the amount they are losing. Building walls will not solve their problem with aliens and immigrants.
They will hire "legal workers", and these guys can come from Mexico, Bolivia or any other UN member nation having good relations with the United States. And the money is going to come from the levies charged on Mexican products. The tax payers are not going to pay for it.