This happens EVERY time I open an existing part and attempt to change the value of my "Cost" custom property from the existing typed-in value to the SolidWorks variable "Calculated Cost".
Even if I delete the existing "Cost" custom property, and create a new one, I will get this error as soon as I select "Calculated Cost" from the pull down.
Works just fine in a new part.
Saw this intermittently in 2013 SP0.0 - See it every time now that i'm on 2013 SP1.0
I created a new costing template and moved the existing templates so that only the new template is available in the pulldown menu in the costing tab.