I'm a bit surprised to see that there may be only a £1 difference in price if you take cruise only ... we've booked Ruby Pricess for the Med in May, and the difference for two of us was £600. Considering the flights actually cost us £200 we've saved a load, even allowing for a couple of taxis to the ship!!!
You need to be far more specific with when and really you will not get an answer on this page. Ring 0800 019 1629 and ask them. You might be surprised to find that there is often only a £1 or so difference with Princess because they charter a large number of their flights and make very little concession if you don't use it. ... Neil.