The Trump administration’s deployment of active duty troops to the U.S-Mexico border could cost between $42 million to $110 million, according to a new independent study.

The Center for Strategic and Budgetary Assessments (CSBA) estimates that it would cost between $112 and $143 per troop per day in operation and maintenance costs for the deployment, officially named Operation Faithful Patriot.

The use of military aircraft in the operation would cost an additional $136,645 per day, according to CSBA.

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The Pentagon is on track to send 7,000 active duty troops to the southern border to stay through Dec. 15, but President Trump said he may order the military to send as many as 15,000 to meet a shrinking caravan of several thousand migrants traveling from Central America.

Trump last month directed the Pentagon to deploy active-duty troops to the border ahead of the midterm elections on Tuesday.

At the low end, the mission could cost $42 million, while the full 15,000 troops could run $110 million, according to the study.

Pentagon spokesman Col. Rob Manning told reporters Monday that the Defense Department has not yet determined the cost of the deployment, but is preparing a cost estimate.

“Our comptroller is working through that process right now, when we get to a point where we can provide a number we will certainly do that,” Manning told reporters at the Pentagon.

The Department of Homeland Security (DHS) requested active duty troops in support of the border mission, but DOD will pay for the operation, Manning said.

Manning would not say why the administration was using active duty troops for the mission instead of National Guard troops, nor would he specify the threat at the border, referring questions to DHS.

Under the Posse Comitatus Act, federal troops are largely prohibited from engaging in domestic law enforcement activities — except in the event of an emergency.

Roughly 2,100 National Guard troops are already at the border as part of Operation Guardian Support, which began in April, and is estimated to cost $182 million, the Pentagon said in May.

Past administrations have traditionally sent Guard soldiers to the southern border, including President George W. Bush, who ordered 6,000 Guard troops to deploy to Arizona, California, New Mexico and Texas as part of Operation Jump Start in 2006 to 2008.

President Obama also ordered up to 1,200 Guard troops to the border in 2010 for Operation Phalanx.

Manning also downplayed the chance that troops would use force at the border, days after Trump said the military should treat rocks and stones thrown by migrants to “consider it a rifle.”

“Our soldiers always have the inherent right of self-defense,” Manning said. “They are well-trained, they are disciplined and they are proficient. ... [When] they have to make a judgment call, our soldiers are disciplined to make the right call.”