According to a recent report, 21% of adults with health insurance

...spent 5% or more of their income on out-of-pocket health care costs (excluding premiums) over the past year.

Source: "New Commonwealth Fund Report: 21 Percent of Adults with Health Insurance Spent 5 Percent or More of Their Income on Out-of-Pocket Health Care Costs," The Commonwealth Fund, November 13, 2014, http://www.commonwealthfund.org/publications/press-releases/2014/nov/21-percent-of-adults