The Trump administration is proceeding with its strategy to build hundreds of miles of new border wall and fencing along the U.S.-Mexico border without first assessing the specific costs of those projects, according to a new report, putting the government at risk of overpaying for ineffective and delayed construction.

In the infancy of President Trump’s term, Customs and Border Protection created the Border Infrastructure Program Management Office. It quickly requested supplemental funds to support Trump’s proposed wall and reprogrammed existing appropriations to contract prototypes. The agency expedited the process to accommodate the White House’s mandate, the Government Accountability Office found in a report released Monday, leading it not to consider costs when choosing locations for new border barriers.

The administration has estimated that it will spend $18 billion to build 722 miles of wall, but that was based on an average rather than the costs of individual projects. Costs can vary significantly in each segment based on topography, land ownership and other factors, GAO said. The slope of the terrain, for example, can drastically alter the price of construction. The auditors previously reported on cost variance based on locations in 2009.