I'm not sure the information on this site is correct, but it seems kind of strange to have to pay twice when buying music. You're paying first for the actual mp3s and then you're paying a monthly fee when what you bought(and want to store in the cloud) exceeds 2GB?
Since the files are the same and there's no need for duplication on UbuntuOne's servers why would they need to charge you for storing them on their servers?