Metrobus fares will be raised March 21 for an estimated 75,000 people who commute during rush hour between the Maryland and Virginia suburbs and Washington, the Metro board decided yesterday.

Distirct fares will not go up at the same time, although a rush-hour increase from 40 cents to 50 cents on July 1 has been officially proposed, with no decision reached.

The suburban increases approved yesterday will be spread unevenly, Virginia commuters will pay from 10 to 30 cents more than current fares,and Maryland commuters will pay from 5 to 20 cents more.

Commuters from Alexandria and most of Arlington who are now charged 75 cents will 90 cents. Commuters from Silver Spring will see fares go up from 75 to 80 cents.

Only minutes after approving the bus fare increase, the Metro board got what members hailed as good news about the subway construction project. For the first time since ground was broken in 1969, the official cost estimate - made twice each year - for completing all 100 miles went down instead of up.

The new figure is a shade $5 billion ago, not counting contingencies - the numerous things that could happen to push the cost upward.

Donald F. O'Hearn, Metro program control director, said the reduction should come from some changed designs, new methods of tunneling and the agreement by Virginia Gov. Mills E. Godwin for the state to do work valued at $33.8 million on the Metro line planned in the median of Interstate Rite. 66 into Fairfax County.

The suburban fare increase will be the first since September, 1975, when the then-novel idea of charging more for traveling in rush hours was put into effect. District fares have remained unchanged since 1970.

Yesterday's decision was reached on a voice vote without discussion, a sharp contrast from the two hours of bitter debate over the same issue that ended in a standoff just one week earlier.

The new higher fares are expected to add about $3.4 million a year to the transit authority's revenues, to help reduce deficits subsidized by local taxation. Wherever the fares go up the most, the tax savings will be the greatest.

However, the increased fares also are expected to discourage about 1.5 million people a year, mostly suburbanites, from riding buses. Last year's total ridership was about 138 million.

William I. Herman, Metro planning director, estimated the loss would be about 5,000 passengers each weekday, the equivalent of about 100 seated busloads.

The fare issue was settle yesterday by an agreement to charge reduced bus and subway fares to all times for passengers age 65 and older and medically certified handicapped riders, starting July 1. District members of the board had demanded that this change be made immediately.

The Prince Georg's County Council, which had opposed the extended half-fare plan, reversed itself earlier this week and made the compromise possible.

At present, elderly and handicapped riders are charged half fare only outside rush hours, which are from 6 to 9:30 a.m. and from 3 to 6:30 p.m. on weekdays.

Under the new plan starting July 1, qualified riders will be able to travel at all hours for half the nonrush fares. For city riders, this means a 20 cent fare instead of the regular 40 cents. For long suburban trips, the reduction in some instances will be dramatic.

Where the new regular rush-hour fare from Reston to downtown will be $1.50 and the off-peak fare will be 70 cents, the elderly and handicapped fare will be only 35 cents at all times.

The new commuter fares that become effective March 21 were proposed last summer and were subjected to two sets of public hearings. It had been planned to put them into effect Jan. 1, but procedural and policy questions caused delays.

Bus riders who testified generally opposed the increase, but suburban government officials who must funnel tax money into Metro subsidies supported them.

The new increases will have the effect of cushioning the impact of even higher fares that are proposed when service on the Metrolmail system is extended by 12 miles, probably on July 1. That line will run from Stadium-Armory station through downtown Washington to Rosslyn and National Airport.

In the main, the bus fare increase adopted yesterday are the same ones that were proposed last summer. The only significant immediate change was the decision by suburban Maryland officials not to raise the off-peak fare for riding downtown. That will stay at 60 cents. The comparable Virginia fare will rise to 70 cents.

The off-peak fares for riding anywhere within Virginia or Maryland will remain at 40 cents.

For Virginia riders, it will cost 10 cents more than at present to ride from Washington across the Potomac River to the "G" (for Government) zone surrounding the Pentagon.

Beyond that, it will cost 5 cents more for each zone line that is crossed. Hence, for a rider going to Zone 4 (west of Fairfax City), it will cost 10 cents more for crossing the river plus four 5-cent zone charges, or 30 cents, bringing the current $1.20 top fare to $1.50.

For Maryland riders, the increase is 5 cents for each zone line that is crossed, with no surcharge for crossing the District-Maryland boundary. The maximum fare of $1.20 thus goes to $1.40.