Include pre-paid, non-refundable costs.
These are the costs that you would lose if you had to cancel on day of departure. Here are examples of those costs and what to include in the trip cost per person.
Airfare: Cost for each person as itemized individually on airline tickets
Accommodations: Accommodation expenses that are not itemized individually are to be evenly split among all travelers
Cruise: Cost for each person as itemized individually on cruise receipt
Rental Car: Assign whole cost to the person who paid for rental (All licensed drivers on rental contract are still covered)
Meals and Entertainment: Cost for each person (pre-paid, nonrefundable only)
Don't forget to include trip costs for any children on your plan, regardless of their age.