But the chief actuary, whose independence is protected by federal law, estimates that the rule proposed by the administration could increase federal spending by $1.2 billion next year and by a total of $38.7 billion over 10 years.

In its latest regulatory agenda, the administration said that a final rule expanding access to short-term policies could come next month. The rule would carry out an executive order signed by Mr. Trump with fanfare in October.

The executive order said the new policies would provide “an appealing and affordable alternative” because they would be “exempt from the onerous and expensive insurance mandates” in the Affordable Care Act.

When he signed the order, Mr. Trump said that short-term insurance would become “much more widely available” and that it would “cost us nothing.”

Mr. Spitalnic confirmed that the new policies would be cheaper for many consumers. He estimated that the full unsubsidized premium for a marketplace plan would be about $600 a month next year, on average — about 75 percent more than the premium of roughly $340 for short-term policies.

But the new short-term plans would also offer less protection to consumers.

Seema Verma, the administrator of the Centers for Medicare and Medicaid Services, defended the president’s proposal.

“Because costs have gone up so much, there are a lot of individuals who can’t afford any coverage,” she said on Tuesday at a Washington Post event exploring the future of health care. For some of these people, she said, short-term plans “potentially could be a lifeline.”