You would need to contact your local council to find out. They will have all the information that you'll need. As for cost, if it's permissible that is, there are a multitude of factors to consider. Size, layout, material etc.
Would need a lot more information to give you even a rough answer.
You will need both Council approval (change of use) and Body Corporate approval to do it.
As to whether it can be done structurally, it depends on the method of construction of the overall building, whether the walls you want to modify are load bearing, the external/internal structure make up and what an engineer has to say.
I would be speaking with an engineer or a builder experienced in apartment renovation as a first step as they can see what is there....
You might also want to look at the BCA for legal requirements for bedrooms- I understand that they must fit a single bed and storage of some description, AND a window.
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