Donald Trump's wall could cost $21.6 billion (£17.3 billion) and take more than three years to build.
That's according to an internal report, seen by Reuters, which analyses the likely cost of the new President's planned perimeter, which the authors suggest will probably be a mish-mash of fences and walls rather than one continuous design.
The report is expected to be presented to the secretary of the Department of Homeland Security (DHS) in the next few days, but the administration may not choose to take the actions it recommends.
The plans say there would be three phases of construction to the wall, covering just over 1,250 miles by the end of 2020.
It also says the American government has sought waivers to address environmental laws on building in some areas, as well as working with existing contractors to plan steel purchases.
The report is seen as the final step before requesting US taxpayer funds from Congress and beginning construction.
The authors assume that funding will be given for the wall by April or May, allowing time for contractors to be found to start construction in September.
Trump has said that Mexico should pay US taxpayers back for the wall, however the Mexican government has insisted it will not pay.
A DHS spokeswoman has claimed the department does "not comment on or confirm the potential existence of pre-decisional, deliberative documents."