Surely the big advantage of a decent sized gas plant is that it can just sit on standby at minimal cost and ramp up quickly on those few occasions when needed. Yes its actual cost of power generated will be higher than if it was used regularly but not as high as what we are seeing here.
By the way where did the power come from to collect those high prices. Was it privately owned batteries, diesel gen sets, coal plants running at 110%, Queensland or load shedding. I assume all the hydro, grid battery and current fossil generating capacity had been fully utilized prior the price spike but that they still profited from it if they could put power into the grid during the event because everyone works on the same price.
