I agree with denas. Estimate. Figure out what you *think* it will cost and insure for that amount. You *CAN* get a refund of the difference if you over-estimate (I have done this more than once). I'd check with the specific company you purchase from, but all the companies I've dealt with so far have said they'd refund the difference. Or you can underestimate & pay more when you find out the total. Remember to only insure costs that are unreimburseable. If you can cancel and get all your money back (say, for a pre-cruise hotel room) then insurance won't cover it.
Again, if you over-estimate, they'll refund the difference. If you under-estimate, they'll allow you to increase the value covered by paying an additional amount. They know that people buy airfare and book hotel rooms, etc, long after putting a deposit down on a cruise, and they are very accommodating for that.
I'm trying to remember, but I think I read somewhere that if you under-estimate, the amount you underestimate by won't be covered for a pre-existing condition (since you will probably pay the extra premium after the 14-21 day period). So it's safer to over-estimate. I'm going to look & see if I can find that again.
ETA: Here it is:
http://www.tripinsurancestore.com/faqs.shtml#f
Just thought I'd mention here again that you can actually usually get a refund of that "extra $10-15 pp". So it's really not worth worrying about.
Sayhello