US Air Force planes conduct an exercise on April 12, 2017, at Kadena Air Base, Japan.
CNN  — 

The Trump administration has been internally discussing how to get countries that host US troops in times of peace to pay more of the cost of keeping those troops stationed there.

The term used to sum up one specific formula under consideration is “cost plus 50.” That means that the United States would work to get countries to cover the full cost of the US military presence in their country, then also pay an additional 50% of that cost. The idea is that the host country gets a certain value with having a US military presence inside their own borders, and they should pay up.

Bloomberg was the first to report on the discussions.

A central pillar of the the Trump administration’s foreign policy has been burden sharing, which has meant encouraging allies to pay a greater share of investment in collective defense. The administration views this policy as a fairer approach than allowing the burden to weigh so heavily on the US government’s shoulders, according to US officials familiar with the idea.

The US currently has a troop presence – in some way, shape or form – in more than 100 countries around the world. Some of the countries that host the highest number of US troops include Japan, South Korea, Germany, Qatar and the United Arab Emirates.

Trump has already pushed NATO allies to put more money on the table to meet the alliance’s 2% of GDP on defense spending guideline, which resulted in $100 billion in new defense spending commitments, said National Security Council spokesman Garrett Marquis.

“Getting allies to increase their investment in our collective defense and ensure fairer burden-sharing has been a long-standing US goal,” Marquis said. “The Administration is committed to getting the best deal for the American people elsewhere too, but will not comment on any ongoing deliberations regarding specific ideas.”

The NSC has asked the Pentagon for some cost estimates on the US troop and military equipment presence in these countries, according to a US official familiar with the matter. But the legality of “cost plus 50” is dubious, the official explained, given that it would include asking the countries to cover the bill for everything from salaries to equipment. With these details and technicalities in mind, it is clear that even if the idea morphed into actual policy, it would take a long time to sort out the specifics.

The exact term has not been used in negotiations with allies, an administration official tells CNN. At this point, they said, it is more of an internal marker or motivator. The negotiations to get allies to pay more for hosting US troops will be handled on a “case by case” basis.

In recent weeks, the US and South Korea reached an agreement on the cost of keeping nearly 30,000 troops in South Korea. The deal, dubbed the “Korea-US Special Measures Agreement,” was signed this week and resulted in the South Koreans paying the US more than $900 million for the next year, an increase of about 8% from what they had been paying.

Those US-South Korea negotiations, however, were strenuous due to Trump’s demands that the South Koreans pay more than before.

The negotiators missed the deadline and ultimately, when they came to an agreement, it fell short of the goal that Trump had laid out by hundreds of millions. That back and forth demonstrates the challenge of getting countries to cough up more money. In some cases, a “cost plus 50” model would amount to more than five times what some countries are currently paying. It could also unleash conversations about whether those countries really want the US troops in their country at all.

Diplomats who have heard the idea through news reports have been stunned, but not surprised. A European diplomat described the idea as a “big deal” and called it politically “incredible” as it would further erode the Trump administration’s relations with US allies.

CNN’s Barbara Starr contributed to this report.