Talk is supposed to be cheap.

And last night at Art and Ann Buchwald's house, it was. The invitation to the casual cocktail-buffet party read: "You Are Invited to Your First Non-Fund Raiser of the Year." About 75 people thought the price was right.

There was, however, a thin thread attached.

The reason for the friendly gathering, also according to the invitation, was "to celebrate the birth of Jack Valenti's New Baby--a book titled 'Speak Up With Confidence.' "

Not much cooing went on about the baby, however, because many of the guests were not that familiar with the book. They were more familiar with its father, the author--now as Valenti, the president of the Motion Picture Association of America, and years ago as Valenti, the speechwriter for Lyndon Johnson.

The guest list, a mixture of media and political names, included Ethel Kennedy; Harry MacPherson, special counselor to former president Lyndon Johnson; Rep. Robert Kastenmeier (D-Wis.); Washington Post Executive Editor Benjamin C. Bradlee; New Yorker writer Elizabeth Drew; Clifford Alexander, secretary of the Army under President Carter; State Department spokesman Henry Catto; Sen. Strom Thurmond (R-S.C.); CBS reporter Lesley Stahl; columnist and reporter Ben Wattenberg; and Los Angeles Times Washington bureau chief Jack Nelson, to name just a few.

Many of the crew meandered from the roast beef and shrimp up for grabs in the house, to the open bar out by the edge of the patio. The small-talk was the majority of the oratory for the evening and the only hype was from the photographers who hauled off Buchwald and Valenti for pictures of the two pals underneath a poster of Valenti.

"If I can get this in the New York Post," cracked Buchwald, "I can write off the whole party."

Other than that the plug for the book was somewhat subtle. Hardly a copy was in sight.

"Do you think I could steal this copy?" Ethel Kennedy asked in a low voice, as she leaned over to a guest standing next to her. She had found one of the few available books on a table outside. She decided against the theft when she saw Valenti walk up. Later, she said, "I didn't have to steal it. They gave it to me anyway."

Instead of stacks of books, the house was decorated with large black and white posters of Valenti, bearing such captions (written by Buchwald) as "I never made a speech I didn't like." They were everywhere. Hard for anyone to miss.

The book, just published, is a guide on how to prepare, learn and deliver effective speeches. But the crowd, which stood a-sippin' and a-munchin', wasn't doing much speechifying. Wisecracks and jokes were the words heard 'round the poolside.

"Hey, where are all the books, Jack?" asked Richard Valeriani, an NBC correspondent and one of the several TV-types in the crowd. "C'mon, I brought my American Express card just for this."

"I wouldn't stoop that low," Valenti said. But he didn't mind telling people to go out and find a copy to buy for themselves.

"If you've got any class you'll go back to Maine and buy the damn book," Valenti said to an old colleague, attorney Hal Pachious.

The title of the book set the mood for the evening--"Speak Up With Confidence." So when Valenti, Pachious and Time correspondent Hugh Sidey started rating the speech-making ability of the presidents, Valenti took his own advice. He gave the man for whom he wrote speeches--Lyndon Johnson--only a B-plus.

The current administration, however, came out top-grade. Ronald Reagan got an A.

The explanation from Valenti: "He's the best in the last generation."

Topping him was FDR, with an A-plus.

John F. Kennedy got a B-plus.. Then came Eisenhower with a B, and Truman, Ford and Carter only pulled in C-minuses.

Who was the worst?

Richard Nixon, who received a D. Said Valenti, "He was plastic and unbelievable, and I'm being totally objective."