The monthly premium for private insurance is dependent on a few factors:
Your age
Your health status at the time of application and
The level of policy you choose.
If you join at a young age with no preexisting conditions, your premium will be lower than if you join when you're older or if you have chronic health problems.
The extent of your policy is reflected in the premium: it can generally be said that you get what you pay for. If you pay more for a premium policy, your coverage will be more extensive.