- 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.
By contrast, 50GB on DropBox will run you $9.99/month (just under $120/year), though it provides a different, less abstracted online storage and sync environment than iCloud does. (It also has 100GB and 350GB+ options.)
You can cancel within 15 days of upgrade or 45 days after a yearly payment. Again, this is in the beta so it's possible pricing and tiers could change by final release time this fall.
Meanwhile, let us know what you think of the prices.