While President Donald Trump has threatened to block U.S. aid to Central American countries that fail to take action against a migrant caravan headed north from Honduras and Guatemala through Mexico to the United States, the cash at risk is a relative pittance.

The real money can be found in the dollars that Central Americans living in the United States, legally and illegally, send home every year.

The numbers are not even close.

“Most of the money given [in aid] to these small Latin American states is not significant enough to have a major impact,” said Matthew O’Brien, director of research at the Federation for American Immigration Reform (FAIR).

A database maintained by the U.S. Agency for International Development (USAID), which tracks all military and civilian assistance by the federal government, shows American taxpayers spent the following sums in these Central American countries in fiscal year 2017:

1.) Guatemala — $229.3 million.

2.) Honduras — $144 million.

3.) El Salvador — $91.2 million.

That is chump change compared with the cash infusion those economies receive from money wired by people in the United States. A 2017 report by the Inter-American Dialogue, a think tank that promotes better cooperation in the Western Hemisphere, estimates that 17 Latin American and Caribbean nations received $75 billion in such remittances in 2017. That was an 8 percent increase over 2016.

The think tank, which based its estimate on data from those countries’ central banks, pegged Guatemala’s share of those remittances at almost $8.2 billion. Flows to Honduras totaled an estimated $4.33 billion, and El Salvador got $5.02 billion.

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Those numbers are “mind-boggling,” O’Brien said.

That money accounts for a staggering share of the region’s gross domestic product (GDP) — 11.5 percent of Guatemala’ GDPs, 18.3 percent of El Salvador’s GDP and 19.5 percent of Honduras’ GDP.

“These countries have become so dependent on remittances that they look the other way on illegal immigration and don’t have a lot of incentive to stop it … It’s really a pathetic economic development plan,” said Jessica Vaughan, director of policy studies at the Center for Immigration Studies (CIS). “It’s not in their self-interest to interfere with the movement of people.”

Not all of the money wired to the Central American nations comes from illegal immigrants, of course. Green card holders and even some citizens send cash to relatives.

But Spencer Morrison, editor-in-chief of the National Economics Editorial, estimated in a post for the Center for American Greatness that illegal immigrants accounted for about $30 billion of the $138.2 billion that the Pew Research Center estimates flowed out of the United States in 2016.

Taking the Inter-American Dialogue’s estimate of the average remittance sent to each country, and the Migration Policy Institute’s estimate of the illegal immigrant population broken down by nationality, it is possible to come up with a ballpark estimate of how much money flowing to the three Central American countries comes from people living illegally in the United States.

The totals are about $1.7 billion to Honduras, $1.8 billion to El Salvador and $4.1 billion to Guatemala.

The “March for the Migrant” caravan left Honduras over the weekend with people who said they were fleeing violence and poor job opportunities back home. NBC News reported that it now has grown to some 4,000 people, including men, women and children.

Trump tweeted Tuesday that he had warned Honduras that he would withhold aid to the country if it did not stop the caravan.

“Until now, Mexico has been perfectly willing to look the other way as long as they knew they would be admitted to the United States.’

The threat seems to have gotten some results. The Guatemalan government this week issued a statement indicating that it would stop the caravan. Authorities in Guatemala also arrested Bartolo Fuentes, a former Honduran lawmaker who helped organize the endeavor.

“Trump is doing what he knows he can do without going through Congress,” said Chris Chmielenski, deputy director of the low-immigration advocacy group NumbersUSA.

Vaughan, who was a foreign service officer at the State Department before joining CIS, told LifeZette that there probably was little Honduras could do to stop the caravan since governments cannot prevent their own citizens from leaving the country.

With the caravan now in Guatemala, Vaughan said, that shifts attention to Mexico.

“Mexico is the key country here,” she said. “Until now, Mexico has been perfectly willing to look the other way as long as they knew they would be admitted to the United States.’

There are signs that may be changing. NBC reported that the Mexican government sent 500 additional police to the country’s southern border with Guatemala.

Related: U.S. Must ‘Do Whatever It Takes’ to Stop New Caravan, Gingrich Says

Incoming Mexican President Andrés Manuel López Obrador, who takes office December 1, has sent mixed messages. On the campaign trail, he vowed not to do the “dirty work” of the United States by stopping Central American migrants.

But NBC reported that the López Obrador said Wednesday that he would give jobs and work visas to immigrants from Central America.

Immigration hawks, however, said that foreign policy can only do so much to solve the problem. Vaughan said Congress needs to close the loopholes that make it easy for illegal immigrants to delay deportation with baseless asylum claims and other policies that make it hard to send them back.

“The real reason they’re coming is because U.S. policy allows it,” she said.

O’Brien, the FAIR research director, made a similar point.

“The first and most obvious thing to take into account, the thing that’s going to stop this sort of thing, is a wall,” he said.