Boston ? Despite exploding costs, most Americans got sizable life-extending bang for their medical bucks over recent decades, says one of the most sweeping studies ever of health care value.

That might come as a surprise to anyone who has ever shuddered over a medical bill, and the report itself raises doubts over how quickly costs have escalated.

However, the study calculated that Americans of all ages spent an average of $19,900 on medical care for each extra year of life expectancy gained over the last four decades of the 20th century. And that cost is worth it, the study authors say.

“On average, the return is very high,” concludes study leader David Cutler, a Harvard University health economist. “But it’s getting worse for … in particular, the elderly.”

The federally funded study by researchers at Harvard and the University of Michigan is published today in the New England Journal of Medicine.

The researchers measured value by the cost of care that extends the average person’s life by one year. The $19,900 spent for each extra year of life – when averaged over 40 years – would be widely considered a reasonable value. Many public and private insurers routinely pay for treatments that cost up to roughly $100,000 for each additional year of life.

The researchers attribute this this relatively low cost for longer lifespan to things like cheap blood-pressure drugs that prevent heart attacks.

However, the study also outlines disquieting trends. It finds that inflation-adjusted costs from birth rose fivefold between the 1970s and 1990s, when the cost of an additional year of life span peaked at $36,300. That means each health care dollar of the 1990s, when expensive drugs made modest impact on cancer, bought a fifth as much real value as 20 years before, when cheaper medicines saved many lives.

Values deteriorated seriously for older people, the study finds. By the 1990s, 65-year-olds paid $145,000 for each additional year of life gained – a value that would be challenged for many individual treatments. These higher costs presumably come largely from end-of-life care that doesn’t extend life very much.