I think you need to see a builder rather than ask. The costs are so dependent on the actual builder, materials required, site slope, soil conditions, access, services, etc etc etc.
As an example - my sister added one room, garage & patio, enlarged & renovated her kitchen, & added a toilet to the laundry. Quotes ranged from $160-300K (double brick). Her unpleasant surprise was that the roof had to be replaced.
The a friend is adding two bedrooms upstairs & a bathroom and she told me she was quoted $80K for brick veneer. The reno has not happened, but I was confused at just how different the prices were. I was skeptical of this price.
Archicentre has costs on a sq metre basis which can give you an idea. And yes, there are always unexpected cost overruns!