"The Phantom Of The Opera," Andrew Lloyd Webber's $8 million megamusical, has been a consistent sellout since its opening last January, taking in more than $28 million at the box office. But it is not expected to reimburse its investors fully until the end of April - more than 15 months into its run.

"Starlight Express," another Lloyd Webber megamusical, has still not recouped its investment (of more than $8 million) 22 months after its March 1987 opening, even though it has taken in more than $42 million at the box office.And when "Jerome Robbins's Broadway" began performances on Jan. 9 at the Imperial Theater, the $7 million musical, which has a record $55 top ticket price, was budgeted to earn back its costs in 63 weeks - but only if it sells out every performance.

Such examples are not unusual these days. The era of the multimillion-dollar megamusical is changing the way Broadway musicals are produced, and creating potential problems for their future, producers and theater owners say.

The high costs of opening a musical - and equally important, of keeping it, and its special effects and elaborate sets, running - mean that investors very often have to wait an increasingly long time to get their money back and to realize a reasonable profit.

"When `My Fair Lady' opened more than 30 years ago, it recouped its investment in 25 weeks," said Rocco Landesman, the head of Jujamcyn Theaters. "That doesn't happen now."

John Breglio, a theater lawyer involved in many Broadway shows, said the problems of the musical theater begin with the skyrocketing costs of the musicals. "What's happened," he said, "is that production costs, which we all assumed just a few years ago for a major musical hovered around $5 million, have often swelled and surged to $7 million or $8 million."

Most people, he said, assumed that the increase was really a function of inflation on costs, materials, salaries. "Well, that's so to a certain degree," he said, "but it doesn't translate into $7 million or $8 million."

"Inflation plays a part," Breglio said. "But there is also the public's demand and expectations of what a musical is. It started with `Cats' - the British influence, the megamusical - and continued with `Starlight,' and `Phantom,' and perhaps `Les Miserables.' The public is demanding so much more in terms of production value, and giving them that kind of spectacle has added a whole new level of costs.

"It's not as if you're just taking an old-fashioned big musical and saying it just costs more today. I think we've now reached a new plateau. That also is to me one of the reasons why you have an upward pressure on the ticket price - because you have an upward pressure on what artists have to deliver.

"You never had this before," he added. "Up until the last several years, the expectation in the musical theater was `music, book, lyrics.' Now it's `music, book, lyrics, spectacle."

Some theater people are convinced the long period of recoupment is hurting efforts to lure investors into the musical theater; others say the recoupment period is still reasonable, but that if costs continue to rise there will be difficulty in the future, especially if the recoupment period grows to a year and half or more.

But most all agree that the predominance and the high cost of megamusicals - and what is perceived by many as the public's demand for more of them - has created many difficulties for the viability of the Broadway musical.

One consequence of the problems is that very few musicals are scheduled to open this season. There are still more than four months to go before the early-May deadline for Tony nominations, but at this point it's not sure that there will be enough new musicals to provide the usual four nominations in the best-musical category.

The critically lambasted "Legs Diamond," with Peter Allen as the Prohibition gangster, which opened Dec. 26, was the first new musical to arrive on Broadway since the short-lived $7 million-plus "Carrie" last May.

More than $5.2 million is riding on "Legs," which came complete with multiple revolving sets and expensive scenery.

"The theater is full of megacrises," said Landesman, whose company owns five Broadway houses and who is a co-producer of "Into the Woods," the Stephen Sondheim and James Lapine musical currently on Broadway.

"But the most alarming of all of them is the difficulty of attracting investment capital. That's getting worse and worse, and the reason it's difficult to attract investment capital is that so few shows recoup, and the ones that do take so long. We have to get costs under control - both capitalization costs and running costs."

Bernard Jacobs, president of the Shubert Organization, Broadway's biggest theater owner (with 17 houses and a half-interest in the Music Box) and a co-producer of the Robbins show and Lloyd Webber's "Cats," said of the growing period of recoupment: "It was a problem when we went from 25 weeks for a musical to 35. And it was a problem when we went to 45, and to 55, and to 65. Each time it increases, it's a problem. But it depends. `Cats' recouped for its investors in 39 weeks, which is good by any standard, and at that point the show cost about $4 million."

Jacobs added that if "My Fair Lady" were to be produced for the first time now, it would cost between $3.5 million and $4 million - "maybe more." Back in 1956, it cost $401,000. And the fastest it could recoup today, he said, would be in 39 weeks, "if you're very efficient."

With the rare exception, he said, "that's pretty much the minimum for a musical these days."

Landesman said that "a big factor in all this is the addiction of Broadway theater owners to spectacle."

"The Broadway theatergoer has also become addicted to getting his $50 worth," he said - $50 is the current top ticket price for most musicals. "The Broadway musical is increasingly becoming a special event, and if you want to go to a Broadway musical once or twice a year you want to see your $50 on stage in a palpable way."

"Not that spectacle assures success," he added, pointing out last season's two most notable failures, the $6 million-plus "Chess" and the $7 million-plus "Carrie."

"It's a cruel business," said Arthur Cantor, a production adviser on "Starlight" and himself a producer and investor who has put his own money into about 20 plays in the last 20 years.

"The use of the word investor is specious. You don't invest in Broadway shows as you would invest in a mutual fund. You invest in Broadway shows as you would in a crap game or a horse race."

Yes, there are many flops, and people can and have lost a lot of money on Broadway. Yet many of them still want to invest. "There's a lot of glamour," Cantor said. And there is always a chance that even if a show doesn't make it in New York, it will still eventually make money.

"You have to think in terms of the long term," Cantor said. "Subsidiary rights: movie sales, touring companies."

And finally, there's always the rare chance of hitting it big. "A Chorus Line," which cost $1,145,000 to bring to Broadway in 1975 - and that includes the cost of the workshops and the Off Broadway run - has grossed more than $135 million for the New York Shakespeare Festival so far.

The producer's profit from the New York run alone - not including road shows and the movie sale - is more than $18 million. That's about $18,000 for every $1,000 invested.

More recently, "Me and My Girl," the British musical currently at the Marquis Theater, which cost $4 million a couple of seasons ago, has returned about 150 percent in profit to its investors, after payment of production costs, according to Ralph Roseman, its general manager.

That's $2,500 for every $1,000 invested, with more to come assuming it continues to make money.

And then there's "Cats," the first of the megamusicals. As of Sept. 4, when the New York production had been running for 309 weeks, it had made $22.6 million profit on an investment of $3.9 million. That's $5,800 for every $1,000 invested.