There isn't due to the JSA (BA/QF offer same fares and service on the kangaroo route, the joint service agreement operating like one airline, but not quite as you have discovered). Other routes with codeshares do not have JSA and there is often 50% difference in price. On the JSA flights it is also possible for the various fare classes to sell out at different rates, also there is a different starting allocation on each side of the codeshare (generally the non-operating airline wants to sell higher priced seats and offers fewer discounted ones). I try to avoid being on the codeshare side, it is rarely advantageous for the passenger.