It's the most expensive because it has the best itinerary (or at least the most unique). The Valor is the cheapest because it is one of many ships sailing that same itinerary. If you had asked in your other post ONLY about cost, I would have told you the Valor is the best bang for the buck. But you also asked about the ports... to me its worth the extra money to have the unique ports. But if you really only care about getting on a cruise ship and having a great time, the Valor is the obvious choice.
It comes down to a supply and demand thing for the cost.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007