Originally Posted by d00t
Ok..... maybe I'm not seeing something.. but...
MEL -> LAX -> MEL - gains 80 SC (Y) (same as from BNE or SYD)
Correct, except that very few people will actually earn full Y status credits on that route. Much more likely to be 40 SC's each way = 80 SC for the return trip in some form of discount Y.
MEL-LAX = 7923 miles according to QF's calculations.
Originally Posted by d00t
MEL -> LAX -> MEL = 110,000 points (X)
Seems as if you get charged extra points for the flight/upgrades... but don't receive any additional SC when paying?
Am I missing something here?

Yes, I think you are missing something. MEL-LAX = 7,923 miles, so the return trip is 15,846 miles. That fits into zone 4 (10,001 to 17,500 miles) range and will cost 80,000 points. See the
QF points table here for zones and costs. This is the same cost as SYD and BNE to LAX return.