Office of Management and Budget chief David A. Stockman has said that the Reagan administration plans to provide "pretty close" to $250 million in federal funds for Metro subway construction next year, an amount in line with the transit authority's long-debated proposal for expanding the rail system to 89.5 miles.

Stockman made the statement at a House subcommittee hearing last month in response to questions from Rep. Steny H. Hoyer (D-Md.), an advocate of expanding the subway system. A transcript of Stockman's testimony was made available last week by a Hoyer aide.

Stockman's comment has been cited by Metro supporters as likely to bolster prospects for the 89.5-mile plan, which includes a proposed Green Line extension to Greenbelt in Prince George's County and a Yellow Line spur to Alexandria's West End.

Federal officials recently began negotiations with Metro officials over a contract aimed at carrying out the 89.5-mile plan, which was designed to be financed at an annual level of $250 million in federal funds.

The transit agency's prospects were jeopardized earlier this month, however, when the Senate included a provision in its sharply debated budget-cutting plan that would reduce outlays for Metro by 25 percent to $187.5 million in fiscal 1986.

The House last week approved a different provision as part of its budget plan, which would provide $250 million for Metro. The Hoyer aide said Stockman's statement may help persuade House-Senate conferees to endorse the House provision.

In his testimony last month, Stockman said, "I believe -- whatever budget numbers are finally in there at the sub-account level -- that we can continue to, in some way, work it around so that we end up pretty close to the $250 million." He suggested that funds might be shifted to Metro from other transit programs.

The 89.5-mile plan is expected to be financed with nearly $1 billion in funds previously authorized by Congress. Metro officials have said they will eventually seek an additional congressional authorization to complete the proposed 103-mile system.

In his testimony, Stockman repeated the administration's opposition to any new authorization for Metro. He said his "reason for agreeing to the $250 million level" was that "the quid pro quo would be a runout" of the federal government's obligation to finance Metro construction.

But he added, "I am realistic enough to know that if the legislators involved want to introduce new legislation and propose additional money in authorization, that is always possible, and that local officials would want to reserve that right for political and other reasons."