A few years ago
DCL started charging a fare for kids under 3 (unless they were the 2nd person in cabin). Kids under 3 paid port charges only. Now they pay a fare plus port charges but I think it is based (as others have said) if they are the 3rd or 4th person in the cabin and their age. I would assume length of cruise plays a part in the price as well. Not sure if type of cabin makes a difference or not.
We are sailing next month on the Fantasy and for a 7 day cruise our 1 year old granddaughters who are the 3rd person in their respective cabins are paying as follows for a Cat 9D on Deck 7 and Deck 8:
Today's Date: 24-Nov-2015
Currency: Rates/Payments are in US Dollars and subject to change
Guest 1 Voyage Fare: $2,177.00
Guest 2 Voyage Fare: $2,177.00
Guest 3 Voyage Fare: $686.00
Vacation Protection Plan: NOT INCLUDED
Taxes, Fees, & Port Expenses: $307.05
So the 1 year olds' fares are $686.00 each and I would assume port fees are about $102 ($307.05 divided by 3 guests)
Hope this helps.
MJ