WASHINGTON - Boosted by the millions he spent looking into the Monica Lewinsky affair, Kenneth Starr's investigation of President Clinton and his wife is about to become the most expensive independent-counsel inquiry ever.

Starr's expenditures totaled $6.2 million in the six months through Sept. 30, when the Lewinsky investigation was going full speed, pushing the cost of the independent counsel's 4 1/2-year inquiry of the president and Hillary Rodham Clinton to nearly $40 million, according to figures released yesterday by the congressional General Accounting Office.

Starr replaced Robert Fiske, who had spent $6 million investigating the Clintons' Whitewater dealings and other matters.

The most expensive independent-counsel investigation to date is Lawrence Walsh's $48.5 million, six-year probe of the Reagan administration regarding its arms and money deals with Iran and its secret war against Nicaragua.

The former Iran-contra prosecutor said Starr's costs would jump by many millions of dollars when his investigation concludes because legal costs for witnesses would be included at that point.

"I'm sure that Starr will exceed me (in costs) before he's through," Walsh said.

Walsh said that under the independent-counsel law, people who were subjects of Starr's investigation but who were never indicted are entitled to reimbursement from the government. Among those who may fit that category are the president - who was impeached but not indicted - and Mrs. Clinton.

Elizabeth Ray, a spokeswoman for Starr, said, "The monumental effort required to conduct the investigation of Monica Lewinsky and others required unusual commitments of resources."

The latest figures are likely to be grist for members of Congress and the legal profession who want to revise the 21-year-old independent-counsel statute when it expires June 30, or even let it die.

"I think the single most effective way to contain costs would be to (have) time limits," said Curtis von Kann, a former independent counsel.

White House spokesman Jim Kennedy said, "Our concerns about the exorbitant expense of the independent counsel's investigation are well-known, but we'll let this latest report speak for itself."

The cost of investigating top administration officials during the Clinton era now tops $76 million, according to the GAO's figures.

Starr's $6.2 million expenditure in the six months through Sept. 30 was up nearly 60 percent from the $3.9 million in the previous six months, the GAO said.

The report does not cover the first 2 1/2 months of Starr's investigation into Clinton's affair with Lewinsky, and does not break out costs for prosecuting Clinton friends Webster Hubbell and Susan McDougal.