The second phase of Metrorail's extension to Dulles International Airport in Loudoun County, US, will cost $3.83bn, according to a current estimate from the Metropolitan Washington Airports Authority.
The authority is in the process of finalising preliminary engineering work to determine the cost of the second phase, which will extend services to Loudoun County.
Phase II, which includes a tunnel and an underground station at Dulles Airport, could cost about $1.3bn.
Earlier estimates had put the cost for the second phase closer to $2.75bn, the cost of the first phase.
The authority is considering alternative alignments and station locations for the Dulles Airport Metrorail Station.
One alternative could bring an estimated cost saving of $640m compared with the current plan for an underground tunnel and station.
Phase I of the Dulles Metrorail Project is 20% complete, while final completion slated for 2013.