This thread has been useful. It's helped me realize that the airfare really will be more than I'm willing to pay & push the total cruise cost into a realm I wouldn't consider worth it.
Therefore I've switched us to the 10-night Mediterranean cruise that sails a few weeks earlier. Even though it's only one night shorter, it costs thousands less, and that difference will likely mostly cover the airfare, which will be a round trip rather than 2 one-way flights, so also less expensive.
I'm really excited about the cruise now, as the travel cost worries are gone, we have never been to the Med before, and I love art and architecture.
Thanks for the honest advice, everyone!