Judging from what you're saying, they seem to indicate that the price is based on per new colour not the number of walls. That should mean that the cost would be $594, not $1000-1500.
Now, going by Dulux's website calculator, a large room would take around 8L of paint and going by Bunnings prices, I worked it out to cost $160 for paint alone, no labour involved. Mind you, this is for 4 large walls, not a single wall.
I recommend phoning painters now and getting quotes. That'll soon give you the answer you seek - and likely some peace of mind about your decision.