This is a known issue that we have introduced with the Q1 2008 version. I can assure you that we have already fixed it and it will be included in the upcoming service pack due in a couple of weeks. The problem appears with expressions that contain brackets. Fortunately you can rewrite your expression in this way:
=PACKING*CARTON*[UNIT PRICE] + PIECE*[UNIT PRICE]
Please accept our apologies for the troubles caused.
the Telerik team