We usually buy one or two photos on each of our cruises. However, we want to hold down the cost and be selective. If you are not careful, you wind up with too many of those $10 - $20 photos. Here is what we do:
If we see a photo we might want, we do not buy it immediately. Instead we have the photo shop save it in a file behind the photo desk. (They do it by room number.) On the last day of the cruise we look over all the photos we have saved and buy only the best ones, if any.