Flat roof to pitched

Hi,
We are wanting to change a flat roof to a pitch roof at the back of our house, on the granny flat extension.
We have a single storey weather board, and have tiles on the main house. The builder has quoted 25k for the change which is 5 m by 11.5m.
Is this a reasonable price?
Also we are extending out the back of the house and will be putting a pitch roof on that part too. By the time they have integrated two new parts they will be taking at least half the tiles off the house. I am wondering about changing the roof to colourbond or zinc alum, not knowing if this will be cheaper or not?
I am also considering this because we had a huge electricity bill this winter so I want to consider insulation matters as well.
Any advice would be appreciated.
Thanks Dave


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