Tables with Different Data

I am using two flat Excel files as my database linked by the key table Code.

One table (Cost) has my costing figures and the other table (Data) is a dump from Quickbooks with my sales figures.

I need to calculate Total Cost by multiplying Quantity by Cost from the Data Excel file by the Cost in the Cost Excel file.

However, in the Code table of my Data file I have elements which are not in the Cost file, such as Credit Note, Miscelaneous, and Sales Discount.

So, when multiplying Quantity x Cost they are eliminated from the calculations.

I tried the following formula WHICH has not worked...

IF {Data_.Code} = "Credit Note"; "Miscelaneous"; "Sales Discount"

     THEN {Data_.Total Sale}

          ELSE {Quantity} *  {Cost}

Any help?

Thank you.