Costs do not have to be shared equally by all POB in order for a flight to satisfy the current definition of 'cost sharing'. That element of the definition is: "the pilot in command pays an amount of the direct costs of the flight that is at least equal to the amount that would be paid by each person if the direct costs were evenly divided between all persons on board".
I know the answer to the question put by the OP (as do you). I just have to go through the drudgery of getting CASA to provide the answer in writing.