From the main site:
Storage capacity
Everyone gets 5GB of iCloud storage for free, and iTunes, iBooks, and App Store purchases aren?t counted against that, nor are Photo Stream images. Mail, documents, accounts, settings, app data, Camera Roll, etc. do count, however. In case they count far more than your free 5GB allow, you can purchase more storage.
10GB for $20/year
20GB for $40/year
50GB for $100/year