"The skateboarders took it the hard way," lamented Louis Johnson, looking back on the year of his life he has invested in choreographing the new movie version of "The Wzi." The production costs of the Universal picture, called by Life Magazine the most expensive musical ever, have been variously estimated at $22 million by the Los Angeles Times, to $28 million by Life, to perhaps $35 million by Variety.

"We had to audition 700 people - they came from everywhere, Chicago, California, Europe - for the dance scenes," recalled Johnson, 48, who was raised in Washington and got his own start in dance here.

"When we told some of the dance professionals they didn't make it, they understood, of course, and took it in their stride. But not these kids - the skateboarders were like 8 and 10 year olds; they didn't know how to deal with it. We could just use a handful from the 130 who tried out, and when you went to explain to the others they were crushed."

"The Wiz." The production costs of the Universal picture, called by Life Magazine the most expensive musical ever, in MGM's 1939 version with Judy Garland, and a hit stage musical), tranforms the Land of Oz into a dreamworld New York City - the film was shot all over the Gotham, from subways and bridges to the World Trade Center. In the update, conceived by film director Sidney Lumet, Dorothy becomes a 24-year-old Harlem schoolteacher and the Munchkins prance around with skateboards, hula hoops, pompons and Frisbes.

The lavish production, the Oz mystique and the cast - which includes Diana Ross, Richard Pryor and Lena Horne - seems to insure blockbuster treatment for the movie, which opens Friday in Washington and many other cities. Whether it will be a hit remains to be seen, and to make a profit it may have to gross as much as twice its production costs, since movie advertising budgets sometimes match the cost of producing the film itself.

Interest in the film is already high, however, and Hecht's downtown department store yesterday some 300 youngsters and oldsters thronged a small salon to watch Johnson put members of the Louis Johnson Dance Theater (who also appear in the film), through some peppery excepts from the Munchkin Dance. In the film, the scene recalls those Busby Berkeley extravaganzas of the '30, in the way the camera recedes overhead to show a myriad of gyrating figures swirling in geometric patterns.

Johnson's career has run a gamut from working with George Balanchine and the Metropolitan Opera to "Cotton Comes to Harlem," but he'd never faced the challenge of choreographing 300 dancers on one production before."

There were no temperament problems among the big names in the film, Johnson maintains.

He's known Lean Horne, who plays the Good Witch Glinda, as an old partment store yesterday some 300 "in her one big number, she had to be hoisted up in the air - she's supposed to be suspended in space - about 40 feet, and it was nothing to her."

Diana Ross, he says, was "really something. I knew she danced, but I had no idea what kind of energy and vitality she puts out. In the 'Brand New Day' number I worked with her one, there she was, jumping on tables, leaping into arms, and never a qualm." Comic Richardy Pryor plans the Wiz, and "the public is going to see an entirely new side of him in this film," Johnson says. "I think they'll be surprised."

Johnson found, however, that his rehearsals kept growing and growing. "We started out with a skeleton corps of 40 dancers. Then it became 80, and then 200 and more. Some of the scenes covered so much space we had to rehearse them in shifts - we couldn't find a place big enough to hold all the dancers at once. The 'Emerald City' scene, which we did out of doors by the World Trade Center had to be shot three times in different colors - green, red and gold. Many of the dance sequences were done in a huge studio in Astoria, but we also shot all over the New York map - in subways, Shea Stadium, the old World's Fair site in Flushing Meadow, all over."

The dance idiom of the Johnson's choreography for the film demands performers of exceptional versatility. "It's a whole range of styles," Johnson says, "semiclassical, jazz, disco - I can' really put any one label on it."

He began to work on the dances, as he usually does, by experimenting with himself and trying things out on his own body or those of his two assistants - Mabel Robinson and Carlton Johnson (no relation). "Before I work with dancers, " he says, "I like to be clear where I'm headed. This becomes especially important in the film medium, which is so expensive and where time is money."

Johnson was hired as choreographer for the film by director Sidney Lurnet, who's seen "Forces of Rhythm," a work Johnson had done for the Dance Theatre of Harlem, and was generally familiar with Johnson's career.

"Lumet was terrific in every way," Johnson says, "he had it together from the start, and the picture is really his creation, top to bottom. He told me up front he's never done a musical before, but said he knew I had the scope - and the eye - for a picture on this scale. He left me alone to do my own thing, but he'd pop in the studio all the time and watch rehearsals and say, 'Louis, you're right on it.'"

Johnson moved with his family to Washington as a youngster and took his first dance classes with Doris Jones and the late Claire Haywood of the Capitol Ballet. "I was an acrobat in high school and they saw me jumping around and saw something - a potential - in me I didn't know about myself. Their training was excellent. Then they sent me on to New York with Chita Rivera, who was also studying with them at the time, and we both got scholarships with Balanchine."

At Balanchine's School of American Ballet, he received what he now calls "the epitome of dance training, a technical core that's always with you." Jerome Robbins cast him in the premiere of his "Ballade," along with Nora Kaye, Tanaquil LeClerq, Janet Reed and other NYC Ballet regulars. Kaye's father, who ran New York showcases for young choreographers, helped push Johnson toward the creative side of the art.

He left the NYC Ballet to dance in such Broadway shows as "My Darling Aida" and "Damn Yankees." Then, on Broadway he created the dances for "Purlie," "Lost in the Stars" and the Scott Joplin Opera, "Treemonisha." The Metropolitan Opera engaged him for "La Gioconda" and "Aida," and his ballets are now performed by the D. C. Repetory Dance Company (of which he was cofounder), the Washington Ballet, the CapitoL Ballet, the Joffrey Ballet, the Alvin Ailey company and other troupes, including his own, established in 1975 and based at Howard University.

After the opening of "The Wiz" next week, Johnson heads for Los Angeles, where he'll be choreographing, and this time directing as well, a new staging of "Tree monisha." He's also considering a couple of film offers - one from Brazil, and one from Dino de Laurentiis, to work on the remarks of "Hurricane."

Johnson also has a show of his own, called "Niggers," that he'd like to see produced in New York. It was first mounted at Howard several years ago, with book, choreography and direction by Johnson. "One critic," Johnson recalls, "called it a black 'Hellzapoppin'." But it's also got a story thread - a young boy who finds himself like Dorothy in the "The Qiz." Broadway needs it - it's warm, human story."

In the meantime, he's content to bask in the trade-gossip optimism about "The Wiz," which, sheerly on the basis of its size and budget, bids fair to make him the most successful black choreographer in history, in box-office terms, at any rate. How does Johnson rate is prospects? "I think it's gonna be a mind-blower," he says. "I think the simple beauty of it is going to get to people."