There is money in the cost side of the business, however, the real money is in the revenue side.
Say, for example, a pax wanted to fly SYD-SIN-KUL return and paid $1800 for the ticket. The SIN-KUL return is (rounded up) 5% of the distance travelled. How would you determine the revenue split between QF and JQ Asia for the ticket?
There are many examples where the revenue is not what would seem logical and it is done for many reasons. However it may further cloud the real performance of a business if those making all of the decisions are not across the detail.