We were just surfing on Costa Website for American travelers and we noticed that fares are quite different depending on your nationality.
A 7 days cruise with Costa Mediterranea leaving on July 13th from Genoa, in the worst cabin (category 1) costs:
for American customers: 1769 $ (including port taxes but not government taxes)
for Italian customers: 1250 euros (=1436 $) including port taxes AND italian state taxes which are 20%. (you can find this information at http://new.costa.it/costa/i/homepage - then choice "destinazioni", "tunisia baleari provenza" , data "luglio" and "13" and finally "prezzi e preventivo")
Moreover they sell a fly add on from NYC at more than 1000 $, which is really a lot...