Cost tab on Constructions Dialog
There are 2 ways to define the cost per area of a construction:
The cost per surface area is enabled the 2-Manual entry cost type is selected. In this case you should enter the overall cost of the construction per area. Make sure to include labour and any other costs directly relating to this cost. When the 1-Auto calculate cost type is used this cell is automatically loaded with the sum of the cost of the individual layers per area and greyed out.