OneCare's retail subscription price, the price charged when you subscribe by purchasing from within OneCare and for renewal is $49.95 in the US for 1 year. During the trial, and sometimes before expiration of an existing subscription, you may be offered a special rate. You must click the link in that notification to receive the rate. I haven't personally seen the offer, but that's how I understand it to work.
You can subscribe or renew with a retail key, often obtaining OneCare for much less than the retail price. Amazon, for example, is selling OneCare for 19.99 plus shipping.
If you wish to cancel you subscription and pick up a retail copy to use for renewal or new subscription, or see if customer service has the ability to credit you for the difference between the offer and the full price if you already paid, contact support.
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
-steve
Microsoft MVP Windows Live / Windows Live OneCare Forum Moderator