Quoted from: http://www.apple.com/legal/icloud/en/terms.html
Cancellations and Refunds
All fees and charges paid by you in relation to the Service are nonrefundable, except as required by law or as otherwise stated herein. You may contact Apple for a full refund within 15 days of an initial upgrade purchase or within 45 days of a subsequent yearly payment. To request a refund as described herein go to http://www.apple.com/support/icloud/contact/ for more details.
You need to contact support via ExpressLane (accessed by clicking Contact Support) or here:
Currently Being Moderated
Oct 18, 2011 9:06 PM (in response to mrdicb)
Go to www.apple.com/support/contact
Select Express Lane for MobileMe (DO NOT select Express Lane for iCloud)
On the next screen, ensure MobileMe is selected
Coose Account & Billing
Next screen choose Cancelling paid subscription; you can provide additional info, optional
Next screen choose Chat; provide requested info
Enter chat session; explain you would like iCloud Storage refund and your within 15 days
Apple rep will ask for birthday, address, phone and last 4 digits of cc used to order storage
You then get your refund
This will be a no charge chat. Shouldn't be this hard, but this should help.