Conservative Treehouse has a description of how Trump could make Mexico pay for his wall. The short answer, a 4% tax on wire transfers of money from workers in the U.S. sending remittances home to Mexico.
Over 10 years this tax would net roughly 10 billion dollars, which the article claims is the projected cost of the wall. Actually, this approach is making illegal immigrants pay for the wall, which seems only fair since they are the reason we need to build it.
