EEP!
This has freaked me out! Everyone said I was over-estimating when I said it would cost us over $100k to extend (we want to build forward on a sloped block, with garage underneath, two bedrooms and a bathroom)... but I'm now thinking that I was being modest and there would be some excavation involved also...

My parents extended 17 years ago - cost $30k for lounge/dining room, bedroom, walk in robe and ensuite.

Another thing to consider - would you want everything to match, or would that not matter? Extra costs could involve painting, flooring, curtains/blinds for the existing part of the house on top of new for the extension ifkwim