Because these are prices paid by insurance companies, many experts say the differences between markets matter more, because they affect insurance premiums that all those with insurance in that area pay, even if they don’t get a blood test or an operation. On average, a cesarean section birth in the Bay Area costs more than three times as much as one near Louisville, Ky., according to the institute’s data.

The average hotel room in the San Francisco area last year cost only around double the average hotel room in Louisville, according to STR, which tracks the industry.

The swing within markets increasingly matters for patients, too, as the share of employer plans with sizable deductibles keeps rising. That means that choosing a provider where your insurance company has failed to strike a good deal could mean significant out-of-pocket costs.

In some cases, prices may be higher because the quality of services or the cost of doing business in a given market is higher. More influential is market power, that of either insurers or hospitals, research shows.

Sherry Glied, a health economist who is the dean of the Wagner School of Public Service at New York University, said a bigger factor was probably how many patients your insurer sent to a given hospital. Popular places are likely to offer better prices, because the insurance company negotiates a bulk discount. The most expensive providers tend to be the ones where the insurance company has little negotiating leverage — and where the service is so rarely used it doesn’t mind the higher price.