During his presidential campaign, Donald Trump promised not only to 'build a wall' to seal the southern US border, but to make Mexico pay for it. Mexico has demurred for now, and in the meantime, the President-elect has instructed Congress to find the funds to build the wall, estimated to cost $10-38 billion.
Nevertheless, a better source of funding is available: the illegal immigrants themselves.
Most undocumented immigrants come to the U.S. to work, and for a simple reason. Wages are higher here. Farm work in the U.S. typically pays $10-12 per hour, while a gardener in Mexico might earn $2-3 per hour, only a quarter of U.S. wages. The differential is even higher in places like Guatemala or Nicaragua, which are substantially poorer than Mexico. For a Guatemalan border jumper, the pay differential can be a factor of 10 or more. That serves as an enormous inducement to come to the U.S.
We can value the size of the inducement through the price of an illegal border crossing. Havoscope, a group tracking black market prices, estimates that human smugglers charge $4,000 to bring a Mexican to the U.S. and $7,000 for a Guatemalan. This corresponds to anecdotal evidence, for example, from a 2010 VOA article:
One [undocumented immigrant] says he paid a Mexican smuggler two thousand dollars to transport him across the U.S.- Mexico border. He walked across the desert for eight nights and slept by day before making his way to Virginia.
If we allow this worker traveled for two weeks to reach his U.S. destination, and that his daily value in the U.S. is about $100, then his total travel cost exceeded $3,000. Crossing the border has substantial value.
We can compare the cost of border crossings with Federal tax revenues actually received from undocumented immigrants. According to the PEW Research Center, about 11 million undocumented aliens reside in the U.S., of which 8 million are in the workforce. A study by the Federation for American Immigration Reform (FAIR) calculates that undocumented workers in 2010 generated approximately $5.3 billion in Federal tax revenues, excluding Medicare contributions.