Calculate a remaining balance through pivot chart

[RaddicusBinx]

Desperation brought me here, as I'm banging my head against a wall trying to figure out why I can't insert a calculated field or item that would allow me to show a running remaining balance for vendors using a pivot chart. I've attached an image to show the basic set-up I've got.

Ideally the running remaining balance would show up under the grand total of accumulated costs/invoices. What step am I missing here, besides being completely stupid?