As I transition from employer-subsidized health insurance to Medicare I'm reminded that not all expenses decrease in retirement. My (and husband's) health care premiums are going from $43/month to about $300/month.
Medicare Part A (hospital insurance): most people don't pay a premium because they've paid taxes while employed.
Medicare Part B (outpatient insurance): $121.80/mo. (for most people; some pay more)
Medicare Part C (Medigap to cover the gaps):
Medicare Part D (prescription drug coverage): cost varies by plan with higher income enrollees paying additional premiums AND you still pay deductibles, copayments, and/or coinsurance. See: https://www.medicare.gov/your-medicare-costs/costs-at-a-glance/costs-at-glance.html#collapse-4811
or Part A & B plus Medicare Advantage (supplemental insurance): https://www.medicare.gov/your-medicare-costs/medicare-health-plan-costs/costs-for-medicare-advantage-plans.html
And... once you've figured out the best plans, you need to go through this process every year as insurers change coverage and premiums!
Medicare costs: https://www.medicare.gov/your-medicare-costs/costs-at-a-glance/costs-at-glance.html
The Squared Away blog shows how Americans' spending changes as they transition from work to retirement. Of course, individual spending varies but the pie charts help by showing the averages. See: http://squaredawayblog.bc.edu/squared-away/housing-health-are-12-of-elderlys-costs/
Read the Medicare Advantage Primer (but note it was published in 2015 and Pat B premiums have gone up to $121.80 in 2016); http://squaredawayblog.bc.edu/squared-away/medicare-primer-advantage-or-medigap/