BIMPROPERTIES - How to apply a formula in Value?

Hi Again,
In BIMPROPERTIES, I created a custom property definition CostperMeter. Still in BIMPROPERTIES, now I want a Value that gets the multiplication of the LinearAxisLength by the previous property.
Using "LinearAxisLength*CostperMeter" or "=LinearAxisLength*CostperMeter" in the Formula field is not working.
Is it not possible, or what is my mistake?
TIA

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