When you call Member Services to book, they will provide you with the cash portion cost for your second person. The first two passengers in a stateroom are priced as adults, regardless of age.
The cash price is a contracted price between DCL and DVC that is agreed upon each year. It will not be the lowest price DCL books, nor will it be the highest. It is usually somewhere in the middle and varies by cruise date.
You will pay the cash portion in full with a major credit card at time of booking, along with a $95 exchange fee.