Once Medicare kicks in at age 65, picking a plan differs little from choosing a plan at work or on an ACA health-care exchange. Different plans offer different coverage, but the Employee Benefit Research Institute says that in general, Medicare covers about 60 percent of the cost of heath-care services, not including long-term care.

Ramnani said that to arrive at an accurate number, her firm estimates what kind of coverage might be chosen by clients and by what they pay out of pocket currently.

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"They might be using out-of-network providers or have specific health-care needs not being covered by insurance," she said. "We assume those expenses will continue."

The biggest costs come from co-payments, deductibles and excluded benefits, along with out-of-pocket costs for prescription drugs and the cost of premiums for Medicare Part B (basic coverage) and Part D (prescription drug benefits). Premiums for Medicare are based on income; the higher your income, the more you'll pay. Beyond basic coverage, there also are other options that come with additional costs.

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