President Trump vowed Saturday to bring the price of his U.S.-Mexico border wall "WAY DOWN!" after a report this week estimated its cost would exceed the amount Trump promised during the campaign.

"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 tweeted. "When I do, just like with the F-35 FighterJet or the Air Force One Program, price will come WAY DOWN!"

In an internal estimate conducted by the Homeland Security Department, Reuters reported Thursday that the cost of Trump's border wall would cost $21.6 billion and take approximately three and a half years to build.

That estimate is far higher than prior costs suggested by Trump and others.

As a candidate, Trump said the cost of the wall would be $12 billion. Speaker of the House Paul Ryan and Senate Majority Leader Mitch McConnell had estimated $15 billion for the project.

Trump's compared his promise to bring down the cost of constructing the wall to his touted success in reigning in the "out of control" price tag of the F-35 Joint Strike Fighter. On Monday the president claimed to have saved taxpayers more than $700 million due to his negotiations on the F-35 with Lockheed Martin. Yet experts point out that the cost savings Trump touted for the 10th batch of F-35s were already expected to occur before Trump got involved and were the natural result of the increased number of aircraft being produced as the production line matures.

Trump also claimed success in talking down the price of the Air Force One replacement after saying Boeing, the company building the jets, was "doing a little bit of a number" on taxpayers.