General Motors Corp.'s promise to pay $10,000 to the heirs of anyone killed in a late-model GM car while wearing seat belts has cost the company $2.4 million during the past 21 months.

But GM officials called the program a success, claiming it has saved an estimated 200 lives in the United States and Canada since it began April 16, 1984.

"Obviously, we would have preferred that no one had been killed" in traffic accidents, said Harold C. L. Jackson Jr., a GM spokesman in Detroit. But the early results of the program indicate that his company's longstanding belief "in the life-saving potential of safety belts" is justified, Jackson said.

The insurance policy, effective for one year after the first registration of a new vehicle, covers accidents in the United States, its territories and possessions, and Canada. Versions of the policy, offered by Motors Insurance Corp., GM's insurance arm, now are being offered by several private insurers and the U.S. Postal Service.

During its first 21 months, the GM plan covered 11.2 million vehicles that were driven 112 billion miles, Jackson said. Included in those statistics are 240 traffic fatalities in which the victims were wearing seat belts. Jackson said 203 of the fatalities occurred in the United States and 37 took place in Canada.

GM said its figures on lives saved are based on its estimate of the number of people who would have been killed in these cars in the absence of the seat-belt campaign. However, the company said it had no specific figures that would allow comparisons between death rates for people wearing seat belts and those who were not.

"That number of fatalities strikes me as being high," said Clarence Ditlow, executive director of the Washington-based Center for Auto Safety, a consumer advocacy group. Ditlow in the past has accused GM of starting the insurance program as a public relations gimmick, one designed to promote seat belts in an effort to get around a federal regulation that would require all auto makers to put passive restraint devices in their 1990 cars.

The July 1984 Department of Transportation rule has an escape hatch: Passive restraint devices such as air bags would not be required in all new cars sold in the United States if states with two-thirds of the nation's population pass mandatory-seat-belt-use laws by April 1, 1989.

So far, 17 states and the District have approved such laws.