An unprecedented behind-the-scenes look at the prices that major insurers pay for medical services presents a serious challenge to prevailing beliefs about how to lower health care costs.

For years, experts have argued that hospital mergers can play an important role in driving down medical spending. Their evidence was data on Medicare spending around the country. President Obama even highlighted exceptionally low Medicare spending costs in Grand Junction, Colorado as an example of a health system that could deliver better prices to consumers.

But new data shows that what consumers and insurers are spending in the private market in a given area doesn’t necessarily align with Medicare spending. Grand Junction, for instance, has higher-than-average private sector health costs. While the area’s large hospitals may drive down Medicare spending by avoiding duplicative costs, according to the New York Times, they often force insurers in the private market to pay more for procedures because of the bargaining power they gain from their size and lack of competition.