For reference, BlueGum’s published starting price is from $134,990 turnkey.
Two things worth understanding about these numbers before you go any further.
Smaller does not mean proportionally cheaper. A granny flat has one kitchen, one bathroom, and one set of service connections whether it is 40 m² or 60 m². Those fixed costs get spread across fewer square metres on a small build, which is why the per-square-metre rate on a studio is usually higher than on a 2 bedroom. Going from 45 m² to 60 m² often adds far less than you would expect, because you are only adding floor, walls and roof, not another bathroom.
60 m² is a legal ceiling, not a design choice. Under the NSW Housing SEPP 2021, a secondary dwelling is capped at 60 m² of floor area unless another planning instrument covering your land allows more. So a “3 bedroom granny flat” in Sydney means 3 bedrooms squeezed inside the same 60 m² as a 2 bedroom, not a bigger building.