Story highlights There's no exact figure for the cost of President Donald Trump's border wall

Republican lawmakers have pushed back on funding the project

Washington (CNN) President Donald Trump vowed Saturday to keep costs down on his proposed wall along the US border with Mexico, although he did not specify how much one of his signature campaign promises would cost.

"I am reading that the great border WALL will cost more than the government originally thought, but I have not gotten involved in ... the design or negotiations yet," Trump wrote in a series of two consecutive tweets. "When I do, just like with the F-35 FighterJet or the Air Force One Program, price will come WAY DOWN!"

The President has not yet released a specific plan for the wall, including an exact cost, though on the campaign trail Trump has cited a $10 billion estimate from the National Precast Concrete Association. But other outside estimates suggest there are enough variables to cost up to $15 billion, and possibly as much as $25 billion, according to a report from Bernstein Research.

The President campaigned on building a wall along its Southern border to stop illegal immigration, and even before occupying the White House, Trump's transition team began discussing plans for the wall with the US Army Corps of Engineers and Interior Department.

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