Why am I charged different prices for my plan?

The price of your subscription depends on the subscription plan you purchased.
Auto-renewable subscription plans are offered with the trial offer. That means that you have a discount on your first subscription and afterward, the full price is to be charged for the new billing period.
A subscription can only be viewed or changed by the users themselves in the settings of their Apple IDs.
So here is an instruction on how to view and change your subscription, provided by Apple Support.
Hope this helps!