Ginger Chapman and her husband, Doug, are sitting on the health care cliff.

The cheapest insurance plan they can find through the new federal marketplace in New Hampshire will cost their family of four about $1,000 a month, 12 percent of their annual income of around $100,000 and more than they have ever paid before.

Even more striking, for the Chapmans, is this fact: If they made just a few thousand dollars less a year — below $94,200 — their costs would be cut in half, because a family like theirs could qualify for federal subsidies.

The Chapmans acknowledge that they are better off than many people, but they represent a little-understood reality of the Affordable Care Act. While the act clearly benefits those at the low end of the income scale — and rich people can continue to afford even the most generous plans — people like the Chapmans are caught in the uncomfortable middle: not poor enough for help, but not rich enough to be indifferent to cost.

“We are just right over that line,” said Ms. Chapman, who is 54 and does administrative work for a small wealth management firm. Because their plan is being canceled, she is looking for new coverage for her family, which includes Mr. Chapman, 55, a retired fireman who works on a friend’s farm, and her two sons. “That’s an insane amount of money,” she said of their new premium. “How are you supposed to pay that?”