Candidate: "We're going to build a wall between us and Mexico. It'll cost us $7 billion."

Electorate: "Umm. That's a lot of money. Let's do something else with it."

If that's how people would react, they should react the same way even if we're getting the money from Mexico. Money is fungible, and as much as our hearts enjoy poetic justice, it shouldn't matter where it's coming from.

There are a couple of situations where we would still want it built, even if its cost exceeded what we were willing to pay ourselves. One is if this is a cooperative, non-zero-sum situation: if Mexico were getting some value from the wall, they might be willing to pay some of the cost, beyond what we can extract via tariffs and seized remittances. The other is if Mexico is terrible at negotiations, which is likely what Trump is assuming.

The break even point is around 60k.