"The gains from eliminating migration barriers dwarf---by an order of a magnitude or two---the gains from eliminating other types of barriers. For the elimination of trade policy barriers and capital flow barriers, the estimated gains amount to less than a few percent of world GDP. For labor mobility barriers, the estimated gains are often in the range of 50–150 percent of world GDP." [1]
A typical example:
"Take a male construction worker in the capital of Ghana. There isn’t much you could do to greatly raise his economic productivity in Ghana; access to better tools or training might make him modestly more productive. But if you let the exact same person emigrate to work at a construction site in any big U.S. city, his economic productivity would rise roughly 700% to 1,000%." [2]
[1] Clemens, Michael A. 2011. "Economics and Emigration: Trillion-Dollar Bills on the Sidewalk?" Journal of Economic Perspectives, 25(3): 83-106. https://www.aeaweb.org/articles.php?doi=10.1257/jep.25.3.83
[2] http://www.cgdev.org/blog/trillion-dollar-bills-sidewalk-why...