If You Use All Your iCloud Storage, Apple Sends You This Email
The email notes that once the storage is full, iOS devices will no longer back up or save data to iCloud and mail sent to me.com email addresses will get bounced.
Users are directed to the iCloud settings on their iOS device to buy more storage. The email also notes that more storage can be purchased on a Mac or PC.
Apple has not released details on pricing for additional storage.