Carnival is/was testing out a plan offering premium steaks and lobster for $18 in the main dining rooms of the Carnival Paradise, Carnival Triumph and Carnival Inspiration. I don't know if they plan to roll this out to the other ships.
The Sensation has no steakhouse restaurant. Except for specialty desserts, milk shakes, specialty coffees, all at the Coffee Bar, all other food is included in your cruise price. That is of course unless Carnival goes ahead with the $18 deal on the Sensation.