[amarkov]

The standard example is Michael Clemens, who argues the total cost of closed borders to the world is in the trillions of dollars. If you want an article reviewing more views: https://www.washingtonpost.com/news/wonk/wp/2016/10/14/why-e...

The basic argument is just the standard free market argument. if A would like to sell their labor and B would like to buy it, and the government intervenes to stop them, that destroys whatever value they could have gotten from the exchange. The fact that there are some border markers between A and B doesn't change that value, although it may introduce externalities.