Hmm ... let me think about how to put that one together.
Tune into your local Spanish TV or radio network on any given day and—if you’re fluent in Spanish, you’'' see multiple ads promising to help you send money home to Mexico. Trump could have the FCC fine networks that air those commercials. Since there's about $25 billion sent from the US to Mexico every year (draining that cash out of our economy), we could set up a fee on those transactions.
We also could enforce "visa fees" on people who overstay.
Plus a new tariff on Mexican imports would pay for it rather quickly since we're their largest trading partner.
If anyone still doubts Trump’s ability to force Mexico to pay for the wall, stop focusing on the cost. The cost of building the wall (estimates range from $10 to $25 billion) is chump change (Trump change?) in comparison to the billions we routinely squander on programs that set back our national security (TSA for one example). When compared to the drain from welfare migrants, Social Security, food stamps, crime, etc, the cost of the “tall, physical, impenetrable, beautiful wall" is negligible.