As a reminder, existing MobileMe customers receive 20GB of additional iCloud storage space for free, through June 30, 2012. This is on top of the 5GB of free storage all iCloud users are getting. After that date, users will be charged for the extra space unless the account is downgraded before that time.
Currently, Apple is charging $20/year for 10GB of extra storage (15GB in total), $40/year for 20GB of extra storage (25GB in total) or $100 for 50GB (55GB total). These additional packages may be purchased (or downgraded) under Settings > iCloud >Storage & Backup > Buy More Storage.
For more information, please visit our original iCloud pricing page.