Did you set this up in advance as a group. You can't do it after the fact.
Its based on cruise fare which is less than the cruise fare you see published. The total fare is made up of cruise fare, non comissionable cruise fare and taxes. The average non comissionable fare is about $250 per person.
If the total cabin cost was $2000. Subtract taxes (I'll use $150pp) then subtract non comissionable cruise fare (use $250pp). This leaves cruise fare of $600 per person.
Your rebate would be based on the average cruise fare of the 8 rooms divided by 2 as you get 1 persons cruise fare for every 8 rooms.