What did he say, actually?
"When during the campaign, I would say 'Mexico is going to pay for it,' obviously, I never said this, and I never meant they're gonna write out a check, I said they're going to pay for it. They are,"
Ppauper, as the High Priest of the Church of Trump, can you provide an exegesis?
I wouldn't presume to so exalt myself.
The president has explained it very clearly, although of course the details are yet to be worked out.
For example, the US had a trade surplus with Mexico before NAFTA, which NAFTA converted into a deficit every single year, roughly $60-$80B dollar a year of late, with the cumulative deficit around $1 Trillion (with a "T") since NAFTA. .
Even though we were promised the opposite by Clinton and Gore when NAFTA was implemented.
In the period from January 1994 through November 2016, according to Census Bureau numbers, the United States ran a cumulative merchandise trade deficit with Mexico of $986,532,000,000.
The new improved USMCA (US-Mexico-Canada Agreement) which superseded NAFTA, should cut that deficit somewhat, and before you know it, the wall will be paid for.
On a side note, there was a study by the Center for Immigration Studies think-tank (using National Academy of Sciences data) that showed that the wall would pay for itself in no time.
According to a 2016 National Academy of Sciences study, the lifetime "fiscal balance" (the difference between what they give to the government and what they receive back) of an immigrant with a bachelor's degree is on average +$183,000, and with more than a bachelor's on average $423,875. That sounds pretty good. The problem is that only 2% of illegal border crosses have more than a bachelor's, with a further 4% having just a bachelor's. 10% have some college, who have a fiscal balance on average of +$41,000, 27% are high school graduates, who have a fiscal deficit of -$69,750, and 57% have not completed high school, who have a fiscal deficit of -$173,375, pretty much the opposite of a Bachelor's degree holder.
Using those figures, the average fiscal drain of an illegal border crosser is something like $74,722, so a $20B wall would need to stop just 267,659 illegals to pay for itself.
The reason there are so few ilegals with bachelor's or higher is that many of them could work in the US legally anyway if they wanted under NAFTA (as could Canadians), so there was no reason to climb through the hole in the fence. I assume that holds true for USMCA which superseded NAFTA.