Your iCloud storage was originally increased from the normal free 5GB for free until 30 September 2012 as a concession because you migrated from MobileMe. Your account storage has now been increased for free until the end of September 2013. At that time if you do nothing your account will revert to 5GB and there will be no charge. If you don't at that time wish to pay to continue to use the extended storage you should ensure that your account usage doews not exceed 5GB - if you exceed it at that time you won't be charged but you may lose some data. (All this has no effect on the amount of storage on iPhones, iPads or computers.) You won't at any point be charged without your specific agreement.
You can purchase additional storage if you wish to, but note that this will over-ride the free storage. The additional iCloud storage levels available are: additional 10GB, $20 p.a., 20 GB, $40 p.a., 50GB $100 p.a. Note that 'iTunes in the Cloud', where such music in your iTunes which was not purchased from the iTunes Store, is charged quite separately from iCloud.
More details in this Apple Help page: http://support.apple.com/kb/HT5527