President Trump and his wife, Melania, flank Japanese Prime Minister Shinzo Abe and his wife, Akie, as they arrive at Palm Beach International Airport on Air Force One. (Joe Raedle/Getty Images)

President Trump vowed in a pair of tweets Saturday morning to negotiate the costs of constructing a wall along the U.S.-Mexico border “way down,” after a government analysis estimated the price at a whopping $21.6 billion.

An internal report prepared by the Department of Homeland Security and reviewed by Reuters this past week estimated that the wall would cost $21.6 billion and take 3 ½ years to construct. DHS spokesman David Lapan told The Washington Post on Saturday that the department is still developing cost estimates and that “any such document is deliberative and pre-decisional.”

Reacting to the estimate, Trump tweeted:

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

Trump sent his tweets from his Mar-a-Lago estate in Palm Beach, Fla., where he and first lady Melania are hosting Japanese Prime Minister Shinzo Abe and his wife, Akie.

During his transition period before being sworn in as president, Trump spoke with executives of Lockheed Martin and Boeing to try to negotiate down the costs of the government’s contracts to build new F-35 fighter jets and a modern Air Force One jumbo jet. After the discussions, both companies, highly dependent on government defense contracts, announced efforts to reduce costs on the programs.

[Initial estimate: Trump’s wall will cost more than a year of the space program]

The border wall is a signature campaign promise of Trump’s. He said on the campaign trail that it would cost only about $8 billion, but that seems to have been an unreasonably low estimate.

President-elect Donald Trump has repeatedly asserted that "Mexico will pay" for his proposed southern border wall – but he's also said the U.S. will be reimbursed by Mexico after building it with taxpayer funds. (Peter Stevenson/The Washington Post)

Trump prides himself on his negotiating abilities. As a real estate developer, he famously squeezed contractors for profits, yet still often experienced massive cost overruns. Government contracting is more prescribed than in the private sector, of course, and involves an open bidding process.