It is all about the price.

Millions of people buying insurance in the marketplaces created by the federal health care law have one feature in mind. It is not finding a favorite doctor, or even a trusted company. It is how much — or, more precisely, how little — they can pay in premiums each month.

And for many of them, especially those who are healthy, all the prices are too high.

The unexpected laser focus on price has contributed to hundreds of millions of dollars in losses among the country’s top insurers, as fewer healthy people than expected have signed up. And that has created two vexing questions: Will the major insurance companies stay in the marketplaces? And if they do, will the public have a wide array of plans to choose from — a central tenet of the 2010 Affordable Care Act?

“The marketplace has been and continues to be unsustainable,” said Joseph R. Swedish, chief executive of Anthem, one of the nation’s largest insurers.

Most Americans with health insurance get it through their employers or from government programs like Medicare and Medicaid. The marketplaces were created under the health care law to give the millions of people not covered in those ways a way to buy health plans.

While major insurers continue to make profits over all, they say that the economics of the marketplaces do not work for them. Insurers can offer marketplace plans at four different coverage tiers, and the government subsidizes the premiums for millions of people. The thinking was that enough healthy people would buy insurance to balance out the costs for the not-so-healthy.

But things are not going exactly as envisioned.