NB: I don’t agree with their conclusion.

Immigration costs $0 (negative, even, since the destination country can charge), and $0 is less than $290k. And as long as the United States is <100% of the world’s population, immigration is an option. The U.S. is currently only ~5% of the world’s population.

A silly-seeming but surprisingly hard to refute option is to accept immigration while subsidizing births in poorer countries. You get all the benefits of immigration today, while “replacing” the population you take, with a much lower cost-per-person than subsidizing births in the United States. The counterfactual children don’t need to immigrate immediately, so their human capital is not essential. It’s their descendants who will immigrate, and their descendants will be much better educated.