Apple has enabled iCloud's Buy More Storage option in the iOS 5 beta beta and here are the prices:
- 5GB: Free (every iOS 5 user will get this by default)
- 10GB: $20/year (including the 5GB free, that's 15GB total)
- 20GB: $40/year (25GB total)
- 50GB: $100/year (55GB total)
So pretty much $10 a month for ever additional 5GB.