InfoPath Dev

I have a InfoPath 2013 Form that has a repeating table. The repeating table pulls information from a secondary data source. The customer access the form and selects a category - which filters the repeating tables nomenclature to items in that category. If the nomenclature selected in the form matches the secondary data nomenclature then the Unit or Measure, ID and Cost populates the default value from the secondary data source. Then the customer inputs the qty they would like of that item - at that time the total cost will calculate to cost * qty. I am trying to add a footer that shows the sum of the repeating tables total cost but the formula sum(my:TOTAL_COST) is not working.

I think you will have to use a filter on your formula for the total cost as well. I've attached a sample for you. Right click the link to the file and select Save As to save locally. Right click the saved file and select Design to open in design mode. Preview the form - I've used default data to ensure there are at least 4 rows when you preview so you can see how the form works.

You were right! that was the missing link once the formula was added to the total cost everything worked perfectly. How would I go about subtracting the qty requested in my forms repeating table from the secondary data share point list that the repeating table pulls data from? I have started with two hidden fields (on hand) & (running total) that pulls the qty from the secondary data source into (on hand) then subtracts the qty the customer enters in the repeating table of the form to result in a running total - I then want that running total to take the place of the (on hand) total of the secondary data source. Any help would be greatly appreciated, I'm sure its a simple solution that I am overlooking from working this form for the past 48 hrs.