Staying in-network doesn't guarantee lower medical bills

These cities are all over the map—and so are their medical prices.

A new study of four common, insurance-paid health services in the 30 largest U.S. cities found not only that those prices vary significantly between those cities, but that there were also radical differences in the prices for the same service within a given city.

In Dallas, for example, a lipid panel, a test for cholesterol levels, could cost anywhere from $15 to $343—a whopping 23-times more than the least-expensive test, according to the survey by Castlight Health Services.

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A CT head/brain scan in Philadelphia could cost $264 or as much as 12-times that amount— $3,271, according the survey, which examined the prices of "in-network" providers in employer-based health insurance plans. In-network providers are, as a rule, less expensive for participants in a plan to use than out-of-network providers.

But "staying 'in-network' does not guarantee low-cost," the study said.

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Castlight Health also found that the average price for a given service in most expensive cities was at least 2.6-times that of the average price for the same services in the least-expensive city.

But the average price of a lipid panel in Indianapolis, the most expensive city in the study for that service, was 4.7-times as expensive at $89, as the average price in the cheapest city, Pittsburgh, where the test averaged just $19.

In addition to lipid panels and CT head/brain, Castlight Health also examined prices of preventative primary-care visits and lower-back MRIs.

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"For every service, for every payer, in every geographic region, we see a large variance for in-network providers," said Dr. Jennifer Schneider, vice president for strategic analytics at Castlight Health. "It's pretty remarkable."

"Many Americans believe that if they select an in-network doctor from their company's health plan they are assured of paying less, or think that health-care costs vary across the country, but not in their backyard. This analysis dispels both of those myths."

The study by Castlight, which works with employer-sponsored plans to lower costs, comes on the heels of the end of the first open-enrollment period for Obamacare insurance plans sold on government-run exchanges. The pricing structure of many Obamacare and employer-provided plans reflects a growing trend toward shifting more out-of-pocket costs for services onto enrollees.

The study makes it clear that depending on where you live or which in-network provider you choose in your city, your annual medical costs can vary widely.

Providers in San Francisco, for example, charge an average of $251 for a routine adult preventative care visit, compared with $95 in Miami. But the cost of the same kind of visit in Phoenix could range from $40 to $195—the biggest variation for that service found within a single city.

And the average lower-back MRI cost $2,635 in Sacramento, California, versus $907 charged by in-network providers in Seattle, Castlight Health found. But in New York City, the same kind of MRI could cost as little as $416, or as much as 11-times more, $4,527—the biggest range for that service, study found.

While the Big Apple was at the extreme for that kind of price variation, Schneider noted that large differences for the same service in a city are far from uncommon.

In Detroit, for example, a lower-back MRI ranged form $938 to $2,240.

One way to avoid steep bills for most of the services examined is to not live in either Sacramento or San Francisco, which accounted for three out four highest average prices. In addition to San Francisco's category-leading preventative care average price, and Sacramento's MRI bills, Sacramento also was the priciest town for CT head/brain scans—$1,404.

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For those who can't move, or aren't inclined to, Castlight Health said customers can keep their costs down with several strategies. Among them is using a national laboratory group for blood tests. It said such groups can charge 90 percent less than hospital labs, and customers have the option of asking for a written lab order and sending it to the lab of their choice.

Customers also should avoid going to a hospital for imaging tests such as x-rays, MRIs and CT scans, which can cost hundreds or thousands of dollars more than if done in standalone imaging center, the company said.

—By CNBC's Dan Mangan