Florence isn't a cruise port, but is served by the ports of Livorno and La Spezia. Together, Livorno and La Spezia serves as the departure port for just fifteen sailings per year from MSC to the Mediterranean.
Overall sailings from Florence are fairly affordable compared to other ports. Sailings scheduled from Florence during the the peak season of summer are much more expensive than the low seasons of spring and fall. Check out the diagram below for the monthly average cost per day (Cruises only depart from Florence between April and November).