The cusomary comment when a great champion retires is to agree that he was a great champion, and perhaps to doubt that his like will ever cross the scene again, and to give assurance that a proper place in history will be staked out for him.

Muhammed Ali qualified for all these things and other encomiums when he affirmed this week that the trail had ended for him; that he would fight no more. He was surrendering without a fight no more. He was surrendering without a fight the title he had won three times. The book was closed, he said. He was 37 and too old now.

There will be skeptics, encouraged by Ali's three previously announced retirements. Always it proved to be that pixie Ali was having his little joke. As for any sneering disbelief of Ali's latest proclamation, it could be rooted also in Ali's use of the three words saying he was officially retired " as of now."

As if to underline his resolve, Ali revealed yesterday from his California home that he had rejected a $50 million offer that would match him with Larry Holmes, World Boxing Council champion, in South Africa.

"I know I could beat that boy, but the wear and tear on my body and other things I'm involved in make that (offer) seem awfully small," Ali said.

Yet in mentioning a purse five times greater than the $10 million that stands as the biggest in boxing history. Ali may cause some wonderment. Is his retirement statement as visionary as his talk of a $50 million guarantee that no promoter could afford?

The overwhelming belief here is that Ali's statement has the ring of sincerity, at least for now. It is known that he despises the drudgrey of training and had for several years. Ali hates getting back into the trenches. Most of all he hates the calisthenics and the roadwork that used to come so easily. To get back into fighting shape, he needs a six-month program. He won't sign up for it.

If there was one development that influenced Ali to make up his mind to quit, it may well have been the out-growth of an exhibition appearance in London a month ago. A British cameraman caught Ali in droopy boxing drawers overlapped by a stomach of Falstaffian flab that appeared to add up to an unsightly 240 pounds or more.

The once-proud Ali, who used to proclaim he was not only "the greatest" but "the prettiest," could now see himself in the morning papers as others saw him, not only out of shape but grotesque. No living man could be more offended. The fact that he retired soon afterward indicates that Ali might have gotten the message that it would be wise to take himself out of circulation.

The factor more likely than any other that could break Ali's resolve to quit in his requirement for a hugh and constant cash flow. He has earned big - about $50 million - but Ali is generous with his considerable army of groupies, and he will learn that the $5 million paydays are not forever.

Angelo Dundee, the longest and closest of Ali's boxing advisers, whose ring strtegies twice saved the title for Ali, said on the telephone from Miami Beach yesterday that he is persuaded to believe Ali's retirement statement. "I think it is of real," Dundee said. But when asked how firm that belief was, Dundee hedged with, "I believe he means it, but I will not be amazed if I am wrong about that."

Ali's dislike for the training grind in the later years was evident in most of his fights. He was unready against Ken Norton, suffering a broken jaw and losing the decision. He geared himself up to lick Foreman in Zaire with his rope-a-dope strategy that suckered Foreman into punching himself out before Ali punched him out in a late round, and he peaked against Joe Frazier in their third battle, the "thrilla in Manila."

But the signs of Ali's deterioration were almost constant, even against supposed pigeons Alfredo Evangelista and Jimmy Young, from whom he took thin-edged decisions that may not have been deserved. He was also humiliated by Leon Spinks, who took his title on a night when Ali was again out of shape. He came back to lick Spinks, but not in one round like Gerrie Coetzee did the other day in South Africa.

Ali gained status as a world figure. He was gladly received in the capitals of Europe and in the Muslim countries, where he is a particular idol. But he was still lusing for more recognition on a political level in his own country when he fought Foreman in Zaire, telling a Washington sportswriter, "Everywhere I go, I am a hero, but why can't I get invited to the White House in Washington? Why don't you see about that?"

His fame led him not to single invitation to the White House but to many. He doted on his trips to Washington for political and social affairs, saying, "Remember, I am a world citizen. I want to do good for everybody in the whold world."

Now, as a retired champion, Ali will be free of the pounding that has been the unhappy lot of the proud man in recent years in the ring. Fighters who couldn't have laid a glove on his hand some head and body in his prime were cuffing him around because the speed was gone. And he will be free of that road work and those brutal calisthenics.

However, as former champ, the White House invitations may not be as frequent and the world acclaim less evident and the big paydays no more. And if his retirement from the ring does not "take," it could be precisely to the degree that Ali misses the adulation, and the multimillion-dollar payday. CAPTION: Picture, Muhammed Ali was in well-fed form a month ago after an exhibition bout with English champ John Gardner. UPI