If you put party size as 4 then total cost will be for those 4. If that makes sense!
(Heartily recommend a commodore cabin if doing a night sailing, it's a proper hotel room with real beds sofas living room area etc and makes things so much more comfortable and bearable than either sleeping in the chairs/on the floor/on a weeny fish finger bunk. Tad more expensive but for us was v worthwhile. Actually even if it was a day crossing I think I'd still book one as it gives you a base to work from/return to if lo's get too hyped up by the on board soft play, I know ds1 did)
Where you off to?