There is a close connection between these 3 terms.
The customers groups will have the following structure:
As you can see, the groups are the ones that are setting the quotas for notes, requests, snippets and attachments sizes.
The price plans are directly related to the groups in the way that each price plan will be assigned to a group. More simple, there is a one to one relationship between groups and price plans.
This approach will allow the administrator to modify the price plans without touching the groups, thus without affecting the quotas of the customers that are already assigned to that group.
The price plans will hold info about the prices and the billing cycle.
The billing cycle will determine the duration of the subscription once the plan is bought.