The biggest price tag of his plan, however, might not actually be possible to calculate. Immigration experts say that even immigrants living in the country illegally still contribute to the economy. A New York Times story from 2013 suggested that immigrants in the country illegally contributed $15 billion annually to Social Security. And the Department of Agriculture has long argued that deporting millions could have major effects on the U.S.’s agricultural economy, as it estimates that half of the country’s farm workers over the last 15 years have been undocumented.

“You can quantify a lot of this. … The part that is harder to quantify is the loss of these productive workers and energetic and innovative entrepreneurs,” says Tamar Jacoby, the president and CEO of ImmigrationWorks USA, a pro-business immigration group. “A pause in immigration would put a huge dent in the American economy that is virtually impossible to quantify.”