The Cuban government is charging the United State $250,000 to renovate the old American embassy building here, one quarter as much as Cuba is being billed for the same amount of renovative work on the old Cuban embassy in Washington, Fidel Castro said last week.

Those figures prove Castro said that the Cuban people are very honest, that American wages are so high that Cubans can do three times the work for the same price, and that he is saving the U.S. State Department $750,000.

Whatever the price, neither Washington nor Havana feels much like quibbling at this point. On Sept. 1, after 16 years without diplomatic relations, the two countries will send 10-person interest sections to each other's capitals.

The delegations will be a far cry from full diplomatic representation, the Cubans will operate under the Czechoslovak flag in Washington and the Americans will be formally attached to the Swiss embassy here. But they will be among the first parts of what Castro's brother Raul described last spring as a bridge, being built "brick by brick, 90 miles from key Wast to Varadero Beach."

The U.S. delegation's arrival will please few people in Havana more than the Swiss. Since the day the United States severed relations with Cuba in 1961, the Swiss have served as consulate, bank, friend and family to hundreds of Americans here, as well as an occasional diplomatic go-between for Washington and Havana.

While the Swiss perform the same functions for the governments of Ecuador, Brazil, Guatemala and Honduras - none of which has diplomatic relations with Cuba, and all of which pay Switzerland to conduct their business here - the United States has taken up by far the most time and man power. When the Americans arrive Sept. 1, the Swiss embassy's foreign interest section staff will be reduced from six foreign service officers to one.

That one will move out of the old American embassy building and back into the Swiss embassy across town.

"The Americans will take over completely," one relieved Swiss official said. "We are very glad."

Among the duties the U.S. representives will take over is visiting several dozen U.S. citizens - some American born some naturalized Cubans - who are in Cuban jails for crimes ranging from espionage to drug-smuggling.

In addition to sending each prisoner a package of medicine and toilet articles four times yearly, "We usually request visits with each of them three or four times a year," the Swiss officials said. But in most cases, he said, the Cubans allow only one visit annually.

The official said he does not know how the American prisoners are treated because "We meet them one at a time, in a small prison office, and never see where they live." All of the visits are conducted in the presence of a prison officer and an English speaking official of the Cuban Foreign Ministry.

The prisoners are aware of human rights protests on their behalf in the United States, primarily through the prison grapevine, but "It's difficult for them to have direct contacts with the outside," he said. The Swiss do not ask the prisoners about their treatment and, in the presence of the Cuban officials, the presence of the Cuban officials, the prisoners "don't dare" ask the Swiss about U.S. protests, the Swiss official said.

The American representatives will also take over the task of distributing interest-free loans, financed through the State Department, to several hundred Americans and Cuban-Americans here who have signed a guarantee saying they will return to the United States when the Cuban government allows it.

The loan applicants, if they can prove need or show that they have been discriminated against here because of their acknowledged non revolutionary loyalties, are eligible to receive between $48 and $144 a month.

Although the American embassy building was offically nationalized by the Cubans some years ago, the Swiss official said, it is still unofficially considered to be U.S. property and neither the Swiss nor the Americans pay rent for it.

What the American delegation will find on arrival is a deteriorated, six story facility, its main doors sealed shut and some windows broken and boarded, filled with dusty, 16-year-old U.S. government-issue artifacts.With the Swiss occupying only a corner of the top floor, the building, located several hundred feet off Havana's main waterfront highway, has developed the musty smell of a long-unused seaside summer cottage.

Two State Department representatives here already arrived in Havana, as have a number of U.S. technicians and workman. Along with Cuban workers, they are installing communications equipment and are painting, repairing and replacing parts of the rundown building.

They have also begun working at the old American ambassador's residence, which served for years as a sort of dormitory for unmarried Swiss embassy staff. Doubtless it will be the new home of the yet-unnamed head of the U.S. delegation. Other Americans in the delegation, like all diplomats here, will live in housing rented to them by a special Cuban government agency.

Maybe part of the reason for the low Cuban renovation estimate is that much of the equipment and supplies needed has been brought from the United States - perhaps the first U.S. made products to reach Cuba legally since the 1962 imposition of the U.S. trade embargo.

One of the first acts of the U.S. delegation will be raise the Swiss flag over the embassy building. In all the years the Swiss have housed their foreign-interest section there, the Swiss official said their flag has never been flown.With the Americans there, he said, the Cubans insisted.