You pay very little for a child under 2 (maybe $50), but you will have to pay the port charges and taxes for her. ; Which will bring the cost up to a few hundred dollars.
One thing to be aware of when travelling with Carnival and a 2 year old is that they do not allow small children (that are not toilet trained) into there pools. ; Even with proper swim diapers. ; Carnival does have a couple of ships that do have specific pools for the little ones. ; We came across this problem last year with DD. ; We were not very pleased with Carnival and Royal Carribean for not disclosing this information a little more clearly in their literature.
Again, make sure they can accomendate your needs with your daughter....