In 2016 the average monthly premium for someone in the 55 to 64-year age group was $580 (which equates to about £5000 per year).
Most plans though include a 'deductible' ( = 'voluntary excess'), whereby the policy holder pays the first part of any claim. The average 'deductible' is roughly the equivalent of about £3000.
It also needs to remembered that the cost of health insurance can vary considerably across different states, with the costs in Alaska typically being well over double those in Utah.
Before you can even start to worry about all that though, you'd need to work hard just to get a residence permit. My friends retired to the States where (because they'd got family connections) they were fully entitled to reside. However it still took them over 2½ years to get through all of the paperwork.