Hi Marcelo,
In Method, you would have to set up a button with actions to loop through your gid/table to update the "total Cost field'. (purchased cost times qth on hand). Not ideal.
In report designer, use the pivot grid object Pivot Grid works like Pivot Tables in Excel. Put the pivot grid in a "page header" or "report header" grouping so you get a good total, not in Detail Report section. Item would go in rows, and PurchaseCost and Qty on Hand in Data, then add a new field to Pivot Grid and add "unbound expression" with your PurchCost * QOH forrmula. Make sure unbound type is "Integer". Save. Pivot will give you a sum of all values. Add Generate Report action to your screen and you have a cost summary of items. Works great.
One of Method folks or I could help you with this if you want support. Hope that helps... Fran