The border wall is a signature campaign promise of Donald Trump's (AFP Photo)

President Donald 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.The Department of Homeland Security said this past week that the wall would cost $21.6 billion and take 3 1/2 years to construct. 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.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.

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.