While it is not an exact science, one source I read estimates that the amount of consumed fuel for a B738 is around 2500 kilograms an hour. An hour extra in the air must come at a substantial cost, and one that neither QF or VA would allow for every day.
The number of flights using SYD, for instance, has doubled since 1985 (the annual number was about 140,000 in 1985 v 290,000 today: divide by two to get the number of flights that landed, or those that departed).
Excess fuel burn must be a substantial cost to the airlines. However it did not seem any flights had to divert to other airports with the BNE problems last night, so pilots are presumably ensuring that their craft have sufficient extra fuel to cope with a delay in the air of this magnitude. Even carrying this extra buffer comes at a financial cost (of course, it may not be able to be avoided).