5 GB of free storage. 20 GB for $20 per annum, 50 GB for $50, and so on, all the way up to a 1000 GB for $1000.
The real story, though, is the fact that MP3s purchased from Amazon are stored for free, and can be streamed via the Amazon Cloud Player.
In other words, Amazon have beaten Apple to the punch in offering music in the cloud.