Florence isn't itself a cruise port, but is served by the ports of Livorno and La Spezia. Together, the cruise port of Livorno and La Spezia serves as the port of departure for twenty cruises anually from both MSC and Costa to the Mediterranean and Europe.
Cruises leaving from Florence during the the peak season of summer are much more expensive than the the low season of fall. On average Florence is fairly inexpensive compared to other ports. Take a look at the diagram below for the average cost per day by month (cruises only depart from Florence between June and November).