During the next billing cycle, you will pay the full monthly price for that configuration. A subscription allows a user to have one instance at that plan level online for one month at a time. If the server is destroyed before that month is up, the user can create another server on the same plan without incurring any additional cost until the subscription renews. At the end of each month, any unused subscriptions will expire.
Subscriptions are prorated through the end of the current month unless there are 5 or fewer days left in the month: In that case, the user is charged the prorated amount for the remaining time in the current month plus the full amount for the upcoming month. That subscription will not be charged for during the next billing cycle.
All subscription prorated amounts are calculated based on the time the user made the purchase. At the end of the subscription period, the user will be charged again for the upcoming month. If the user has no active servers, they will not be charged anything.
Any unused subscriptions are prominently displayed to users on the new server creation screen: