Medical coverage obviously costs more the older you get, but once you reach Medicare-eligibility age, your actual monthly cost could go *down* in retirement. There are still supplemental and Rx plans for Medicare recipients, but that probably won't cost you as much as premium + deductible. I just asked my dad and my parents were paying about $15,000/year in premiums for the two of them and my sister when she was under age 22 (my mom's employer doesn't provide health insurance, and my dad had retired before age 65 and started his own business). But then my sister left the plan, my dad turned 65, and the cost of the supplemental medicare plan is about $270/mo plus about $100 for the medicare, or $370/mo. Currently they are paying about $500/mo for my mom's coverage, but she'll turn 65 this year, and then that will go down to $370/mo. total. It seems to be good coverage; their Rx costs are $3-4 for a generic, and about $30 for a non-generic. When my mom broke her wrist last year, it cost them virtually nothing (maybe $500) since the plan coverage was good.
So for planning purposes, just get quotes for the current cost of insurance for whatever-age-year-old you want to retire (for me, for example, I could look up on my state exchange's website how much a plan would be for a 43-year-old) and then factor in inflation and add that to your budget. You can even figure that you'll pay the maximum deductible amount, or the maximum out of pocket, for a margin of safety.
Currently, DH and I are on separate plans. His employer pays most of the cost of his coverage, and he pays $134/mo. for a $3,000 deductible, and the employer puts $1,000 into his HSA for him. My employer coverage is expensive, so I opted to shop on the state exchange (Minnesota's is called MNsure), and I got a $2,750 deductible for $153/mo. We are nearly maxing out our HSAs this year, as well, but are unlikely to spend everything we put in, so that will just be savings for the future.