It must be purchased prior to when penalties accrue. If you choose a non-refundable deposit (i.e. the "Best Price" fare), you must buy the plan immediately. Some long cruises have penalties due earlier than final payment date, and the CPP must be bought before that day. But, for a 7-day cruise with a refundable deposit, CPP (Platinum or Standard) will need to be purchased before final payment.
You can add it for one person in a cabin, but if that person cancels before sailing, the person remaining will be charged "single supplement".
The refund for cancellation will be refunded to the credit card and quite quickly. We were refunded on the Standard plan at just a week!
I don't have an answer for your ballpark!