There are three plans:
$20 a year, which gets you an extra 10GB of storage (totaling 15GB)
$40 a year, which gets you an extra 20GB of storage (totaling 25GB)
$100 a year, which gets you an extra 50GB of storage (totaling 55GB)
In the U.S. Apple is offering a full refund of extra storage purchases as long as it's canceled within 15 days of an upgrade or 45 days after the annual subscription is renewed.
How do these prices stack up to the ones Apple was offering with MobileMe? An extra 20GB of storage on top of the 20GB that came with MobileMe ran $49. An extra 40GB cost $99 a year, making Apple's new plans slightly more expensive per gigabyte. However Apple is now offering the initial 5GB free of charge, versus MobileMe's $99 per year that came with quadruple that amount from the get-go.
How much data most users will need remains to be seen, since Apple has not launched iCloud integration in the existing version of iOS beyond offering cloud backups of purchased music, apps, and TV shows. As part of iOS 5, which launches this fall, Apple's iCloud service will play an important part in the backing up of various iOS devices, as well as small chunks of application data, like progress on games and books. The service will also keep backups of photos and videos from iOS device camera rolls, device settings, and how apps are organized on the home screen.