A supporter of Donald Trump stands across the street from a protest held by a number of Latino organizations outside of NBC Studios in New York City on Nov. 7, 2015. Andrew Renneisen/Getty Images

Republican presidential candidate and presumed GOP nominee Donald Trump has frequently promised the American public that, should he win the White House, he’ll make sure that all undocumented immigrants in the country get sent back to their home countries no matter what. However, a new study shows the undertaking may cost much more than expected.

Deporting everybody living illegally in the United States and preventing every future unlawful entry would cost between $400 billion and $600 billion over the course of 20 years, according to a report published Thursday by the American Action Forum. It would also reduce the U.S. real gross domestic product by more than $1 trillion in that same time span, shrinking the economy by 2 percent.

The study found that roughly 6.8 million undocumented employees worked in the private sector in 2012, making up 5.6 percent of all people employed in the country. Deporting all of them would significantly cut the country’s workforce and cost private industries somewhere between $381.5 billion and $623.2 billion annually.

“The things Donald Trump has said are utterly unworkable,” Douglas Holtz-Eakin, the forum’s president and top economic adviser to Arizona Sen. John McCain’s 2008 presidential campaign, told Reuters.

To put that estimated price tag of hundreds of billions in perspective, the United States military budget in 2015 — which is by and large the heftiest chunk of U.S. discretionary spending — was roughly $640 billion (compared to $519 billion for nonmilitary discretionary expenditures). That’s no small sum for a military. The United States’ military expenditures nearly match the combined armed forces budgets of China, Saudi Arabia, Russia, the United Kingdom, India, France and Japan, according to the Peter G. Peterson Foundation.

U.S. Department of Defense Spending | InsideGov

Trump, for his part, does not agree with all of the report’s findings, including the notion that the country’s economy would shrink by up to 2 percent if all undocumented immigrants were deported.

“I saw that report and they don’t even have it right,” Trump said on CNBC when asked about the findings. “We certainly don’t want to shrink our economy.”