Michael,
I am not completely sure about this and I have not tried it, but I think it is not possible to perform such mathematical operations on values that are not simple numeric values but values with a unit of measurement (like Length, Area, Volume type of data). So I would first try to convert them to the "Number" Data Type using the "STRTONUM" Data Conversion function, and then try to perform those multiplications.
Or, maybe another option would be to create a new Property with the Data Type "Area", and with a constant value of "1 m2". Then divide the Area value with this value because I think you can divide an Area value by another Area value, and the result will be a Number value. Then you can multiply the result with the price/m2 value.
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