Looks to me like the county contributes to the cost.  $151 monthly isn't much, but it's something. 

"IMPORTANT: Medical premiums displayed are before the County's contribution of $151.00. You must be enrolled in a County medical plan to receive the County contribution."

https://www.slocounty.ca.gov/Departments/Human-Resources/Employee-Benefits/Retiree-Benefits/Retiree-...