It is hard to come up with counterfactual tariff revenue if NAFTA is repealed. You need to go through the U.S. uniform tariff schedule, apply those tariffs to the current duty-free trade with Mexico, and then estimate how much trade volumes would decrease if tariffs were reimposed.
Here is a back-of-the-envelope estimate. In 1991, tariffs on imports from Mexico came to around $567 million on $39 billion of imports. Imports from Mexico are now around $294 million. That would imply revenues around $4.5 billion. A slightly more (but not much) sophisticated estimate uses the tariff rates from 1991 and adjusts for the share of imports using USITC data. At the two-digit level (which is a blunderbuss) that would get you revenue of $3.9 billion today.
Of course, the Uruguay Round then cut tariffs by about half, so assuming that Mexico would retain normal trading relations after the U.S. repealed NAFTA, then those revenues would fall to something around $2.0 billion.
Even if you cut that by half again for falling trade volumes, you get $1 billion per year. Enough for the wall?
One engineer estimated the amount of materials needed. A second estimated the cost. Cut that materials cost in half (for only 1000 miles of wall) and you have $8.5 billion. Double that again for total cost: $17 billion.
Given low U.S. borrowing costs, the Drumpf administration could plausibly claim that ending NAFTA did indeed force Mexico to pay for that wall, even if some of that tariff incidence fell on American consumers. (Once NAFTA is finished, President Trump could try to get Congress to remove normal trading relations from Mexico and raise tariffs further, or impose taxes on wire transfers from the United States, but that is not something he could legally do on his own.) Over two decades or so, those revenues could finance a wall. If you squint really hard. And Donald sure is squinty-eyed.
The things I think about these days.