With estimates of gross earnings ranging as high as $50 million, next month's Sugar Ray Leonard Thomas Hearns welterweight title fight in Las Vegas appears likely to become the biggest money maker in boxing history, eclipsing last fall's $30 million Larry Holmes-Muhammad Ali bout for the World Boxing Council's heavyweight championship.

"When you have someone out there that people believe to be a legitimate challenge to Ray Leonard, people are going to want to see it," says Michael G. Trainer, Leonard's attorney. "Ray biggest fight so far was Leonard-(Roberto) Duran I. That grossed about $24 million, of which Ray got $9.7 million. This fight will break that record. It will be the biggest fight that's ever been done."

Trainer said he expects the fight to gross between $35 and $40 million, less than the highest estimates but still sufficient to break all records.

Three weeks before the Sept. 16 fight, Caesars Palace in Las Vegas, which purchased site rights to the bout for an estimated $4 million, reported that only 3,500 seats remain available in a temporary, $1 million, 25,000-seat arena being constructed specifically for the bout.

Within days after the fight was announced at a July 7 press conference in New York, fans snapped up all the available $500, $100 and $50 seats, leaving only 7,000 tickets at $200 and $300 remaining by the first week in August.

"This, so far, has been bigger and better than Holmes-Ali," said Pat Cruzen, a Caesars staffer. "For most of our fights, the tickets don't really sell until a week before the event."

Scheduled on a Wednesday, the fight is expected to attract thousands of high-rollers to Las Vegas, not just to Ceasars Palace but to other casino-hotels, for both the weekend before and the weekend after the event. A number of casinos have purchased blocks of tickets to distribute to their better customers, and, before the week is out, the Las Vegas Chamber of Commerce estimates those customers will spend between $100 million and $150 million at the gaming tables, restaurants, cocktail lounges and elsewhere. $1

At Capital Centre, where the fight will be shown on closed-circuit television, more than 15,000 tickets at $30 a seat have already been sold. "The response has been phenomenal," said a Centre spokesman. Similarly, the Spectrum in Philadelphia already has sold half of its 18,272 seats at $25 a ticket and expects a sellout by the week before the fight.

Capital Centre and the Spectrum are two of between 250 and 300 closed-circuit outlets in the United States and Canada where the fight will be telecast to an audience estimated in the neighborhood of 1.5 million. At an average of $20 a ticket, that may mean a gross of $30 million, of which 45 percent would go to the facilities telecasting the fight and 55 percent to the fight promoters.

"So far, knock on wood, everything's gone just right," said Shelly Finkel, the New York-based rock concert promoter who teamed up with lawyer Dan Duva to put together Main Event Productions, the organization promoting the Leonard-Hearns fight. "I myself waver back and forth as to who's going to win. And we had the blessing of a baseball strike this summer."

Finkel said Main Event has guaranteed Leonard $8 million and Hearns $5 million for the fight, although it appears virtually certain that both fighters will walk away with a good deal more than their minimum guarantees.

Under terms of the agreement, once Main Event has cleared expenses and a reasonable profit, Leonard and Hearns will then be entitled to a 25 percent share each of the remaining profits.

"You've got to protect yourself against the home run," said Trainer.

Probably the leading financial question of the fight involves the relatively new pay-per-view television capability, a form of cable television. Available mainly in California, Massachusetts, Ohio and Texas, pay-per-view will beam the fight live directly to homes that $15 apiece for the privilege.

"Now, compared to June of 1980 (the time of the first Leonard-Duran fight), we have so many more homes with the pay-per-view capability," said Trainer, estimating that there are currently between 800,000 and 1 million homes with such capability.

Duva, Main Event's president, has estimated gross profits from pay-per-view receipts may be as high as $15 million, although Finkel said that estimate represents "the optimum projection."

In previous fights, said Trainer, Leonard has attracted between 45 and 50 percent of the potential pay-per-view audience. If that holds for the Hearns fight, the pay-per-view gross would be closer to $7 million than $15 million. Of the $15 that each subscriber pays for home telecast of the fight, the transmitting station will get $5 and the fight promoters $10.

Outside North America, the fight will be telecast live to about 75 countries throughout the world under terms of a separate set of negotiations being handled by Bob Arum of Top Rank Inc., another fight promoter.

Hearns, the World Boxing Association champion with a 32-0 record and 30 knockouts, and Leonard, the World Boxing Council champion with a 31-1 record and 21 knockouts, spent early August touring the country, promoting the fight. Both are now in training.

Meanwhile, in Las Vegas, Leonard said he wouldn't spar for the next two days because of a welt under his right eye.

Leonard suffered the injury in sparring Tuesday. He skipped ring work Wednesday but the wound worsened yesterday when he resumed sparring.