The ''wall'' that President Donald Trump proposes to erect along the US-Mexico border would cost the American tax payer $21.6 billion over a period of three and a half years, an internal report of the US Department of Homeland Security (DHS) says.
This is far greater than the figure of $12 billion cited by Trump in his election campaign and the $15 billion projected by Republican House Speaker Paul Ryan and Senate Majority Leader Mitch McConnell.
According to the DHS report, the first phase would be the smallest, targeting sections covering 42km (26 miles) near San Diego, California; El Paso, Texas; and in Texas's Rio Grande Valley.
The plan is to seal the border in three phases of construction of fences and walls covering just over 2,000 km (1,250 miles) by the end of 2020.
The second phase of construction proposed in the report would cover 151 miles (242 km) of border in and around the Rio Grande Valley; Laredo, Texas; Tucson, Arizona; El Paso, Texas and Big Bend, Texas.
The third phase would cover an unspecified 1,080 miles (1,728 km), essentially sealing off the entire US-Mexico border.
With 1,046 km (654 miles) of the border already fortified, the new construction would extend almost the length of the entire border.
The DHS report also factored in the cost of acquiring private land, one reason for its steep price increase compared to estimates from Trump and members of Congress.
Bernstein Research, an investment research group that tracks material costs, has said that uncertainties around the project could drive its cost up to as much as $25 billion.
The report assumes DHS would get funding from Congress by April or May, giving the department sufficient time to secure contractors and begin construction by September.
Trump has said Congress should fund the wall upfront, but that Mexico will reimburse US taxpayers. Mexico has said it will not pay.
The report will eventually be presented to DHS secretary John Kelly in the coming days, although the administration may not necessarily take actions on it recommendations.