adding 'separator' parameter to Plan#price_description
Adding 'restrict_with_exception' to Plans > Subscriptions assocation.

Bad things will undoubtedly happen if you delete a Plan which users are already subscribed to. Make it hard
to do accidentally.

If a Plan in a real-world app really needs to be deleted, a better option would probably be to 'archive' it?
(Add an 'archived' boolean, default false, and set the default scope to 'archived: false')