I don’t understand why you have two repeaters, but you would want to perform the calculation in the repeater in which you want to see the result–so the repeater you show in your last screenshot.
There are two ways to do this: (1) use column values and perform calculations in the OnItemLoad event, or (2) perform calculations only on the repeater item (e.g., the minus and plus buttons). In this recent thread, @UXProtoTyper nicely demonstrates these two approaches and explains benefits and drawbacks of each.