Have you received any bills from Apple?
Apple always send bills via email, when they take money from your CC. In these emails it also states what the amount is for.
Anyway, this is just a thought, but sometimes when first signing up for paid services, they reserve a small amount of money from your CC to check, whether the details are correct and your cc is working. Companies such as Three and PayPal us this technique, perhaps Apple does the same. In any case, if this is what goes on, then the money is not actually deducted, but rather reserved, i.e. it is still on your account, you just can't use that amount for the time of the account checking.