Does using iCloud cost you money?
The first 5GB of storage on iCloud is free, after you use up your free storage space it's $20/year for another 15GB. iCloud wont cost you anything unless you want more than your free space- it prompts you to buy more if you use all of your free space.
what jasonicus said
how can I tell how much space I am using and what is in there? Mail, documents, calendars etc