Talk, in this political city, has usually been a cheap commodity. But tax payers may come to think differently if more people like Robert O. Tiernan are appointed to high federal office.

Tiernan was a congressman from Rhode Island who lost his seat in 1974 to Edward Beard, a house painter. After his defeat, Democratic hierarchs in the House consoled him with an appointment to the Federal Election Commission, a job that pays $50,000 a year. In 1976, congressional friends consoled him again, writting legislative language that lets him continue a sideline law practice, Tiernan also finds time to serve as president and treasurer of the Rhode Island Reds, a professional ice hockey teams.

Juggling these three jobs, commuting regularly to Rhode Island and keeping up with all the friends he has made in politics naturally requires heavy use of the telephone. In less than two years, 1,688 long distance calls were billed to Tiernan's government credit card. The charges exceed $2,750, not including tax. All were paid by the election commission, which enforces political fund-raising laws.

"Sure, I did make a lot of phone calls," Tiernan says. "A lot of people call me and I have to talk to them . . . I'd have to check them all to be certain, but most, if not all, would be strictly business calls."

Some calls billed to Tiernan's government credit card appear to fit this description. But from July 1975 to May 1977, Commissioner Tiernan also billed the United States for 57 calls to his law office in Rhode Island. He charged to the credit card precisely 100 other long distance calls to his home, and he called his beach house in Narrangansett 59 times at taxpayers expense.

Besides, Tiernan let the government pay for all sorts of other long distance conversations that clearly seem personal - to his physician, to real estate agents, to assorted friends, to the National Democratic Club, to old cronies in Congress. One day last March he called the Rhode Island Reds four times, and while golfing at the Congressional County Club in May, he kept calling Thomas O'Halloran, part owner of the hockey club.

The 48-year-old Tiernan explains his energetic telephone life by saying he must stay in constant touch with the election commission when he is practicing law in Providence or when he is at home in Warwick so that he can deal with sensitive compliance problems, such as issuing subpoenas. "I've also used the credit card to return calls from Capitol Hill or from someone in a political campaign" who is seeking advice about the election laws, he says.

To the office of one Democratic Senator, for instance, Tiernan placed 84 long distance calls. Those were in response to official inquiries about filing campaign fund-raising reports, Tiernan contends. But the Senator wasn't running for re-election at the time and wasn't raising money. And an aide to the Senator, describing a longtime friendship with Tiernan, says she received most of these calls, along with others after hours at home.

For a time, too, Robert Tiernan Jr., the commissioner's son, charged obviously personal calls to his father's government credit card.

Young Tiernan studied journalism at the University of Rhode Island. Among calls charged to the government are 27 to Night Life magazine in Pawtucket. In addition, government telephone records reflect 15 long distance calls to the university, 56 calls to the young Tiernan's former residence, 18 to a motorcycle shop in East Greenwich, R.I., 64 to Ronald Loparto, a friend, and 50 more to James Sheehan, another friend.

Commissioner Tiernan says he didn't authorize his son to charge calls to the taxpayers, "and when I found out about it, I told him he couldn't do it anymore." But the elder Tiernan hasn't reimbursed the government for personal calls, whether placed by him or by members of his family.

A few people who frequently received Tiernan's government-financed calls back his claim that official business was discussed. One friend, Robert Frulla, a golfing companion who is executive vice president of the Frieght Forwarders Institute, says 29 calls to his office from Tiernan can be explained by the institute's desire for high-level advice about organizing a political fund raising committee.

"Who else would you go to, if not to a friend?" Frulla wonders. "I could go into hundreds of official things I've asked him about . . . I could go on and on for hours and hours."

To some extent, all this extra cost can be blamed on Congress. Theoretically, in accord with a 1976 law reorganizing the election commission, each of the six $50,000-a-year members is supposed to serve full time. "Members of the commission shall not engage in any other business, vocation or employment," the law declares.

But in a Senate-House conference report that accompanied enactment of the reorganization law, Tiernan's political friends arranged a little-hoted exemption-specifically "as an accommodation to me," he confides. The conference report, interpreting congressional intent, says the prohibition against outside business activity applies only to those commissioners who devote "substantial" time to it. In other words, part-time law practice or part-time management of a hockey team can be permissible.

By one measure - the number of phone calls from Rhode Island to the election commission - Tiernan spends much time away from Washington on personal business. Toll records show 101 such government-paid long-distance calls on 77 different working days in 1976.

Happily for Tiernan, Democratic friends in Congress left the definition of "substantial" entirely to his flexible discretion. He asserts that "my time and efforts have been devoted fully to the work of the commission."