The report makes clear that the wall President Donald Trump promised during his campaign would be considerably more expensive than the fencing favored by most border-control experts.
Many congressional Democrats reacted strongly to the news of plans for the wall and its estimated price.
Mexican President Enrique Peña Nieto, who has vowed that his country will never pay for the wall, cancelled a scheduled visit to the United States amid the flap, although he and Trump spoke by phone on January 27 and agreed not to publicly discuss funding for the border barrier.
He also can not retreat on his repeated insistence that Mexico will pay for the construction, a promise that was largely viewed as a fantasy even before the cost of the construction was revealed to be far higher than Trump originally claimed.
The longest and the most challenging of the phases would be the third phase in which the border wall will stretch for another 1,080 miles. A lot of people say, 'Oh, oh, Trump was only kidding with the wall.' I wasn't kidding.
Foreign Minister Luis Videgaray will meet U.S. Secretary of State Rex Tillerson and Homeland Security chief John Kelly, authorities here said Wednesday.
Reuters said the $21.6 figure is contained in a U.S. Department of Homeland Security internal report the news organization viewed on Thursday.
So what is Trump's own administration saying about the border wall's construction? Mexican President Enrique Peña Nieto has said his government has no intention of paying for it, and many taxpayers are anxious that the bulk of the cost will fall on the American people's shoulders.
"The department does not comment on or confirm the potential existence of pre-decisional, deliberative documents."
During his campaign, Trump also characterized Mexican migrants living in the U.S. as murderers and rapists and pledged to build a wall that he said Mexico would pay for.
According to the report, the first phase would cover 26 miles near San Diego; El Paso, Texas; and a part the Rio Grande Valley in Texas. I built a wall along Israel's southern border.