The amount of times I get asked this is unreal but it is actually the question you should ask!!!! After you find out who you are asking.
Are they a registered builder
Do you know of another person they have done work for as a reference
Where are they based
How long have they been in the industry and the list goes on,
Personal referral always trumps 3 free quotes off the internet from some one you've never met and who has no idea of the area and people.
So back to the question, How much??
Each job is different. the construction of the building or the age of the property to accessibility for materials, Are you on a Budget or do you want gold plated fixtures with all the trimmings. All these are factors which influence the price.
Then there are simple basic hourly and sqm costs for labour rates.
Attached at the end is a link to a cost guide for 2019, I know its not up to date but hey this is free.
It should help a little, but in the end you need a builder to come out, listen to what you want and be advised on what is possible and for them to give you a quote on the agreed works.
Hope this helps,