Apple today revealed the pricing details of much awaited iCloud service. The first 5GB of cloud storage is free for all and if they exceed the storage limit they can upgrade the limits with one of the three paid packages.
An additional 10GB of iCloud storage, apart free 5GB, can be purchased at $20 per year; 20GB at $40 and 50GB at $100.
Well if you need more than 55GB of storage on iCloud there are not direct options to purchase, may be you have to Apple support team.
By the way you want to know how much iCloud storage space you have used so far, check out it this post for instructions.