Double HAH for GJ too......... having lived in one for 9 weeks as we waited to move into our new house it was pants.
To put some actual figures on the table it cost us $1350/sqm to build well above "Minimum building code" standards.
Money was well spent on insulation, during the wee cool snap we've had recently overnights we've not needed any heating on at all.
House a comfortable 12-16 C degrees above outside temps.
If we'd put a second floor on top that would have worked out at $1650/sqm for the second floor