Promoters say they expect tonight's Sugar Ray Leonard-Thomas Hearns welterweight title bout in Las Vegas to gross between $35 million and $40 million, making it the richest fight ever.

But it appears unlikely that the gross will reach $50 million, a figure bandied about earlier this summer.

And there are skeptics who say the fight may not even top the $30 million record for last fall's World Boxing Council heavyweight championship bout between Larry Holmes and Muhammad Ali.

Lou Falcigno, the nation's largest closed-circuit television exhibitor with 40 outlets in the New York area, said the time of year and the midweek date will hold down crowds.

In September, Falcigno said, fight fans are "not going to jump out of their comfortable easy chair in front of their television set, drive 10 miles, park in some dark parking lot, plunk down $20 or $25, sit in some arena that's semifilthy and watch a fight. They'll do it in the beginning of the summer because they're looking to get out of the house."

Falcigno's 40 closed-circuit outlets are among 275 throughout the country which will be showing the fight at an average price of $20. Shelly Finkel, the New York-based rock concert promoter who teamed with attorney Dan Duva to put together Main Event Productions, the organization promoting the fight, said ticket sales are going very well.

With closed-circuit attendance expected to be about 1.5 million, revenues could reach $30 million at the $20 average ticket price. But there are indications that sales have lagged in recent days after a surge during early summer and midsummer.

At Capital Centre, where almost half the 19,000 seats had been sold by the first week in August, 1,500 remain unsold. At the University of Maryland's Cole Field House, only about 1,300 seats had been sold by the close of business yesterday.

Capital Centre controls 47,000 seats -- at the Civic Center in Baltimore, Towson State College, Cole Field House and Capital Centre -- and fewer than 20,000 are available. Elsewhere in the Washington area, the fight is being shown at the D.C. Armory and at Charles Town Race Course.

At Caesars Palace in Las Vegas, which purchased rights to the fight for approximately $4 million and spent another $1 million building a temporary 25,000-seat arena, a few $300 tickets are available after an initial buying surge caused speculation of a sellout by mid-August. Prices in Las Vegas range between $50 and $500. The live gate should amount to $5 million, but Caesars expects far larger profits from the money left at its gaming tables by a high-rolling fight crowd.

Pay-per-view television, available mainly in California, Texas, Massachusetts and Ohio, represents the other major source of revenue for the fight, with best estimates for the gross ranging in the neighborhood of $7 million.

There are between 800,000 and 1 million homes with pay-per-view capability, which enables them to receive the fight at home for a single payment of $15. Of that, the transmitting station will get $5 and the fight promotion $10.

Leonard, the World Boxing Council welterweight champion, has been guaranteed $8 million for the fight. Hearns, the World Boxing Association champion, has been guaranteed $5 million. Once Main Event Productions has met expenses and cleared an agreed-on profit, each fighter will be entitled to 25 percent of the remaining profit.