For many years the U.S. taxed cash transfer services such as Western Union which is heavily used by illegal immigrants to send money home. It will be very simple to put this tax back into place.
Migrants from Mexico and Central America are sending home $69 Billion per year. Nearly all of it done via cash transfers rather than EFTs via banks since illegal immigrants don't have bank accounts generally. Put a tax of 0.1% on each transfer, and you would raise $690 million each year. This would raise $13.8 Billion over 20 years - ample to pay for most of the wall if technology is used in conjunction with concrete.
Mexicans In The U.S. Are Sending Home More Money Than Ever
http://www.npr.org/sections/goatsan...the-u-s-are-sending-home-more-money-than-ever
Migrants from Latin America and the Caribbean are sending more money to their families back home than ever before.
These annual "remittances" — as they're called by analysts —
topped $69 billion in 2016, according to central bank data compiled in a
new report by the
Inter-American Dialogue, a Washington, D.C.-based think-tank. The money has been a lifeline for the national economies of many countries in the region since at least the 1990s, when
Manuel Orozco, a political scientist who authored the report, first began tracking remittances. They climbed steadily since then, only to plummet when the Great Recession hit the U.S. economy in 2008. But they began to rise again in 2012. The 2016 tally is the highest amount on record and an increase of nearly 8 percent over 2015.