The posting of 40SCs is perfectly correct for a Y class CHC-SYD and the upgrade costs look pretty normal. If you paid $2200 it is quite feasible for the flight to be in Y. Checking atm A Y fare r/t CHC to BKK comes in at NZD1908 and an L r/t AKL to SIN at NZD1722 . Taking half of each fare gives a price of NZD1815. Add taxes and fines of NZD465 and it would be NZD2280

I don't think that there is a mistake in what you paid or what you have received

It would only have been another NZD213 for Y class each way and been able to have low priced upgrades on those too