Francy Eagan of Lexington Park, Md., was adding up the cost of attending next week's presidential inaugural when she paused and said, "You must keep in mind that this is a celebration. I worked 15 months for this, and it's worth every penny."

For Mrs. Eagan and her husband, Rex, every penny will total more than $500, and that doesn't include her black chiffon gown with the beaded and sequin top that her mother is making, and other clothes that she got as Christmas presents. Despite the cost, the Eagans are among the chosen few. Inaugural Committee spokesmen said last night tickets for all events are sold out or soon will be.

The question of whether tickets remain for specific events is complicated by the fact the invitations permit the purchase of varying numbers of tickets. The nine inaugural balls, all of them white tie except for the youth dance will be limited to 40,000 people and inaugural cochairman Robert K. Gray has vowed that "even my own mother won't be able to get a ticket" once capacity is reached. But 44,000 invitations were mailed out across the country, qualifying each recipient to buy from two to eight tickets at $100 ticket or $2,000 for a box for eight.

"It's a nightmare" trying to keep track of the total ticket requests, one committee employe said. A worse potential problem exists for the inaugural variety show gala next Monday night at the Capital Centre, for which 80,000 invitations (maximum two tickets each) were issued, after orginal planners had wanted to limit the invitations to 40,000. The Capital Centre seats about 18,000.

The Eagans have shelled out $200 for a pair of tickets to the inaugural ball at the Smithsonian's Museum of Natural History. Each state is assigned to a specific site -- Virginians will dance at the Air and Space Museum. District residents, led by Major Marion Barry and Councilman Jerry Moore, the lone GOP official at the District Building, will dance at the Washington Hilton.

The Eagans also will spend $200 for three day's lodging at the Embassy Square Hotel and still more for an open house in their suite there; $10 for a ticket (limit of one) to the Distinquished Ladies Reception Monday morning at the Kennedy Center; $20 for two tickets to the reception for Vice President-elect George Bush Monday afternoon at the Museum of American History (she gets a third one free because she is serving as a hostess; their daughter, Cindy, 16, will get to go to that event); $20 for two tickets to the Governor's Reception at the Sheraton Washington Sunday afternoon; and miscellaneous costs, including dining out.

Despite that busy schedule, the Eagans are passing up more events than they are attending. They are not going to the $100-a-person gala at the Capital Centre, and are still waiting to hear from their congressman about attending the actual swearing-in of Ronald Reagan as president (free if you can get a ticket) at noon Tuesday at the West Front of the Capitol.

"We talked to his office last week, but we haven't heard anything," said Mrs. Eagan, who, as the Republican chairman of St. Mary's County, acknowledges the irony of being at the mercy of Democrat Roy Dyson, a hometown neighbor who defeated her candidate, incumbent Republican Robert E. Bauman.

The Eagan's chances of getting tickets from Dyson are slim -- each member of the House was alloted 21 seats, plus 95 standing spots. Senators got 28 seats and 200 standing spots, and as of last night "all of the tickets have been distributed," said Tom Decker of the Joint Inaugural Committee.

Moving the ceremony to the West Front of the Capitol (from the traditional site on the East side) will permit more people than ever to see the swearing-in, but the demand still is larger than the supply, partly because it is one of the few events for which there is no charge. "Beware of someone trying to sell a seat to the swearing-in," Decker said.

A total of 24,574 seats and 115,000 standing passes have been issued. In addition to Congress, major recipients were President-elect Reagan (2,500 seats and 30,000 standing) Vice President-elect Bush (600 plus 6,000), the Inaugural Committee (600 plus 2,250) and the Republican National Committee (600 plus 1,250). Former members of Congress, presidential electors and federal judges are eligible for two tickets each.

Some reserved seats for the parade, which follows the swearing-in, were available last night in all price categories ($10 to $100, depending on location along the route, which generally follows Pennsylvania Avenue from the Capitol to the White House). Tickets not sold to invited guests by Friday will be sold to the general public from a stand at the National Visitors Center at Union Station.

Mrs. Eagan has discovered that helping her candidate win the presidency didn't mean the end of her volunteer work. Last week, she was at the nine-bedroom Georgian colonial home of Barbara Taylor in Potomac, addressing envelopes and licking stamps, just as if the campaign had not ended. But this time the envelopes contained prized invitations to inaugural events.

Most of the people who got invitations got the "commemorative" version, which are free, but are not good for admission to any events. One Maryland Republican said that Marylanders got only "a pitiful [small] number" of even the souvenir invitations, but added , "I guess that's because Carter carried the state."