Metro's budget committee members, facing the high probability they will have to raise fares on the regional bus and subway system next year, began discussing yesterday just how high to go.

Budget analysts briefed the committee on the impact of fare increases ranging from 5 cents to 15 cents above the current 85-cent base fare.

But the committee and its parent board are months away from reaching a decision about whether or how to raise fares and how to adjust the formulas that establish the costs of rides that are longer than those covered by the base fare.

Metro carries about one million riders a day, and they pay a variety of fares depending on what time of day they ride, where they get on and where they get off.

The District, Maryland and Virginia have somewhat different philosophies on how much of Metro's operating costs should be paid by riders and how much should be picked up by general taxpayers in the form of subsidies. As a result, fare-setting discussions are agonizing and time-consuming.

"It's fair to say it's likely there will be a fare increase," said board member Matthew S. Watson of the District. "I don't think we raised fares enough last time." Fares last went up in July 1989, when Metro raised the minimum fare by a nickel. That was the first increase since 1984.

Like all public transit systems in the United States, Metro doesn't make enough money from fares to cover its operating expenses. When all is said and done, about half of the $612 million operating budget comes from passengers, the rest from federal, state and local subsidies.

When fares went up last year, the board said it would consider another increase in 1991. Several local governments have urged the transit agency to look at fare increases because local governments were having trouble paying rising subsidy costs. The District recently missed its quarterly subsidy payment and Metro had to borrow $25 million.

Watson and board member Gladys W. Mack said yesterday they worry that governments will bear a disproportionate responsibility for financing the Metro system even as governments themselves are struggling. Meanwhile, they said, Metro riders have gotten off relatively easy in the last 10 years.

The budget committee also approved a plan to establish child-care centers near Metro stations in Virginia and Maryland. The full board is expected to consider the plan Sept. 27.

Originally proposed by Rep. Frank R. Wolf (R-Va.) as part of federal legislation, the centers would serve children of commuters and Metro employees.

Metro would provide free land near the station, while local governments would build and operate the centers.