In Dallas, a 15-mile trip can save a patient $12,000 on joint replacement surgery.

Coloradans who come down from the mountains for treatment in Denver can save $19,000.

And in Maryland, a 9-mile drive from Baltimore to the suburbs can save $36,000.

Hospitals sometimes just a few miles apart get paid wildly different prices for hip or knee replacements in much of the U.S., according to an analysis of Medicare data.

The public health insurance program for Americans over 65 spends $7 billion on more than 400,000 joint replacements each year.

A study of Medicare’s hospital data shows 30 cities where the agency’s average payment for uncomplicated joint replacements at the most expensive hospital is at least twice as much as it is at the cheapest.

The chart above shows the agency’s average payments to hospitals for joint replacements over a year. The variation in costs among individual surgeries is potentially much greater.

Medicare last week announced changes to the way it pays for joint surgeries, noting that “the quality and cost of care for these hip and knee replacement surgeries still vary greatly among providers.”

The new payment method will begin to make hospitals accountable for the costs of rehabilitation and complications for 90 days after the operation.

The aim is to incentivize better care by penalizing hospitals if patients suffer complications and rewarding those whose patients recover swiftly.

The changes build on an experimental program called bundled payments, which Medicare has tested with willing hospitals.

The approach will now be mandatory for 800 hospitals in 75 metro areas, and the agency expects it to bring savings of $150 million over five years.

The new payment won’t level the differences among what neighboring hospitals charge. But it’s intended to begin rationalizing a payment system that often defies logic.

The goal is to make sure hospitals get paid more for actually providing better care.