Even if all these machinations caused Mexico to cough up cash, the U.S. Congress would need to appropriate the money to build the wall. “It is likely that the vote for funding construction of a border wall would be the biggest, most consequential, and hardest-fought since the passage of Obamacare,” Geraghty writes. “[U]ntil all of these obstacles were overcome—until funding was procured from Mexico, the Congress gave its approval, and the courts signed off—construction of the wall couldn’t even begin. Structural engineers argue that, pulling out all the stops, the wall could be completed in four years. Trump’s signature promise likely couldn’t be fulfilled until mid-way through his second term at the earliest.”

O’Neil told me that hiking visa and border-crossing fees for Mexicans seemed like “the only way you could really potentially [finance the wall] and have [the U.S.] remain compliant with treaties that we’ve signed and other agreements.” The revenue that a President Trump raised from this scheme “could go to promoting security on the border, which could be a wall.” In this scenario, Mexicans, not the Mexican government, would be helping pay for the wall, if indirectly.

But imposing those fees would have consequences. Mexico is the top destination in the world for American tourists, drawing around 26 million visitors per year from the United States, or more than four times the number of unauthorized Mexican immigrants currently living in the U.S. Trump could raise the costs for Mexicans visiting the U.S., O’Neil said, but in response “you should expect the Mexicans to raise the costs for Americans going there.”

Trump, of course, could merely be staking out an extreme position at the outset of negotiations with Mexico—a common deal-making tactic known as “anchoring.” But, as Aaron Wallen, a lecturer at Columbia Business School, recently told Slate, Trump’s inflexible stance on how the wall will be financed, and the Mexican government’s rigid refusal to foot the bill, appear to have left nothing in the way of what negotiators call a “zone of possible agreement,” or ZOPA. In instances such as these, additional issues are usually folded into the talks. Mexico, for example, might agree to subsidize the wall in return for a new trade deal with the U.S. that is favorable to Mexican interests. Trump would be able to say he’d made good on a key campaign pledge, but he’d pay a pretty penny for the privilege.