The report also includes information for more than two dozen states that declined to set up their own marketplaces, leaving at least part of the job up to the federal government.
Premiums will vary significantly depending on an individual's income, where she lives and what type of coverage she buys. A 27-year-old in Fairfax County, Va., for example, could spend between $124 and $258 on a health plan, depending on how robust she wants it to be.
A family of four in Fairfax County that earns $50,000 could get a health insurance plan with no premium at all, because the federal tax credit would cover the bill.
Most people using the marketplaces will have incomes low enough to qualify for a government subsidy. A recent administration report found that 56 percent of the roughly 41 million uninsured people eligible for the marketplaces could pay monthly premiums of $100 or less.
Health experts say it is a good sign for consumers that premiums have come in lower than expected. Under the law, the plans must offer a basic set of benefits, including mental health and maternity care, which previously were not included in many private plans. Insurers are also forbidden from rejecting or charging people more because of pre-existing conditions.
Many experts worried that those factors would drive up the cost of insurance. They partially credit competition on the marketplaces, where people will be able to directly compare plans from different insurance companies, for restraining premiums.
But they warn that premiums don't tell the whole story.
The low rates are possible in part because insurance companies created special plans that include fewer in-network doctors and hospitals than many current plans.
This may not be a problem for healthy people who currently lack insurance. But those with illnesses may discover that their specialists are not covered by an exchange insurance plan. Low-income people accustomed to a certain community clinic may find that going there is no longer an option. And everyone may encounter long waits to see a doctor.