How can I sign up for a non-free iCloud account (having exceeded the 3 free ones)
Why not just upgrade one of the existing accounts?

http://support.apple.com/kb/PH12796

or see this for deleting unused iCloud accounts

http://www.macworld.co.uk/how-to/mac/how-remove-your-apple-id-completely-shut-down-your-itunes-store-icloud-other-accounts-3474388/

(too much info to précis here, unless votes/comments would suggest it worth it)