Jan,
It depends on a couple of factors. Price matching will only occur if the same criteria exist. Same Ship, Same Sailing, Same Cabin Class - it's an apples vs. oranges thing.
What site did you find the cheaper price on? If it's an online travel agency, then it may be a group space rate which is $50-100 less per person than the standard individual rate that one would get from Carnival Reservations, or a PVP.
DS - 
Cruise Specialist
ShipVacations.com
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