There are always extra costs. Expect roughly an extra $300-$500 for incidentals and spending cash. For spring break students, the ideal lines are NCL, Carnival or Royal Caribbean. Carnival has a minimum age of 25(unless someone 25 or older is in the stateroom), while the other two have a minimum age of 21. I would likely recommend Royal Caribbean as it has fewer extra costs than NCL and more activities than Carnival.