THEATERS CUT COSTS AND CROSS FINGERS

A new plan to lower the cost of producing - and of seeing - shows in several underused Broadway houses will be put to the test for the first time tomorrow night when the musical revue ''Stardust'' begins performances at the Biltmore Theater.

The League of American Theaters and Producers has been negotiating for more than a year with each of the 14 craft guilds and unions represented on Broadway to make those theaters - the Belasco, the Ritz, the Nederlander and the Lyceum, along with the Biltmore - more attractive to producers. The five theaters were designated ''endangered'' for reasons that include location on the periphery of the Broadway district and second balconies that have proven difficult to fill at modern ticket prices. In exchange for a producer giving up some potential box-office income by reducing ticket prices in those theaters by about 20 percent, the unions have agreed to various concessions, from lower wage scales to more flexible work rules.

''The whole idea of this is to keep theaters open,'' said Berenice Weiler, who, along with her partners Marilyn S. Miller and Barbara Carrellas, is providing general management services to the ''Stardust'' production.

Actually, the idea was to encourage the production of serious drama on Broadway, according to Harvey Sabinson, executive director of the theater league. Drama has been the chief victim of ever-increasing production and ticket costs on Broadway. ''Our goal was to bring back to Broadway some of the producers who were going elsewhere because expenses were so high,'' Mr. Sabinson said. The endangered-theaters plan technically is a six-month concession, after which the show, if it is successful, would roll over to a regular Broadway production code contract.

The endangered-theaters plan is the latest among several attempts to control costs on Broadway; it replaces the ''499-seat plan,'' in which, for similar concessions, certain theaters closed off their balconies and sold seats only in the orchestra. (Ironically, the last show to play at the Biltmore was the short-lived ''Honky Tonk Nights,'' presented last summer under the 499-seat plan.) Producers felt that theatergoers were uncomfortable in half-closed theaters and sought another solution. A $1 Million Musical

''Stardust,'' a revue of songs written by Mitchell Parish with such composers as Hoagy Carmichael and Duke Ellington, opened in November at Theater Off Park, a nonprofit Off Off Broadway company, to enthusiastic notices. Four producers and two associate producers raised $1 million to move the show to Broadway - less than one-fourth the going rate for musicals and one-eighth of what it will cost to open ''Starlight Express'' at the Gershwin on Feb. 26. ''Stardust'' has a cast of six and a nine-member orchestra.

From the outset, the producers were committed to maintaining the show's intimacy and to keeping ticket prices down. That led them to the Biltmore. The serious-play objective of the endangered-theaters plan was the first problem they had to overcome. Several key contracts, including those with the unions representing musicians and actors, have not yet been negotiated by the league, leaving the general managers to fend for themselves.

They met with a team representing many of the unions with which they would be negotiating, and got off to a good start. ''The consensus was that something should be done to help get the show on,'' Ms. Weiler said. Since the musicians' union hadn't figured in the endangered-theaters plan, the general managers met with it right away. ''The musicians had to be convinced first,'' Ms. Weiler said, ''and nobody else would agree to anything before them.''

''Management told us they were going to reduce their ticket prices,'' said John Glasel, president of Local 802 of the American Federation of Musicians. ''It's a bit of an inconvenience for us, because each time we make concessions, we have to have a ratification meeting. But we're all interested in seeing the theaters lit. And if the show is successful, we expect to go to full contract.'' Lower Break-Even Point

When it opens, ''Stardust'' will have a top ticket price of $37.50, or $10 below the top price for most Broadway musicals. Half of the 270 seats in the second balcony will sell for $10, a point of pride for the producers. ''The unions were very supportive and helpful,'' said one of the producers, Louise Westergrad. ''Rather than cut the seat prices across the board, they allowed us to have a $10 balcony.'' Lower prices, she hopes, will help return some spontaneity to theatergoing, and increase the chances for the show to develop the kind of repeat audience it had downtown.

Lower costs have meant somewhat lower risks for the producers, as well. According to Ms. Weiler, ''Stardust'' will have a potential weekly gross of $213,600 and a break-even point of 62 percent of that, or about $132,400. ''If we were not 'endangered,' '' Ms. Weiler said, ''the break even would be closer to 80 percent.''

All of those in the profit pool - including Albert Harris, who conceived ''Stardust'' four years ago and is its director, the designers, Mr. Parish and the composers or their estates, and the producers - also will be getting reduced amounts until the show pays back its investors.

''If we can do a musical on Broadway for $1 million, we're doing pretty well,'' said Ms. Weiler. Though it is not the serious drama for which the plan was intended, ''Stardust,'' if it succeeds, may restore investors' faith that smaller-scale shows can make it on Broadway.

''Everyone came together for the sake of the theater,'' said Burton L. Litwin, another of the show's producers. ''You don't go out of this show humming the scenery and the laser beams. We think that if this works for us, it will help bring some life back into the theater.''