Hi Parrot Mom. $100 pp sounds high but you can expect to pay at least $75pp Here is a link to a service Rome Airport Shuttle Discount Shuttle Rome Airports
which was recommended to us by people who had used them with great success. What we wound up doing was making arrangements through our hotel (Lancelot Hotel) near the Colosseum for around 90 euros for 4 of us. You might want to try your hotel to see if they can help you.
I agree with Kuki that generally the cruise line air is more expensive but once in a while they offer flights that are very reasonable.
Have a great cruise! You will love Roma!