First In, First Out subtraction from rows?

[dee-u]

Hmmm, reminds of the "net-worth"-calculation we have in the company i work for.
Basically, we do it as follows:
Each stock-location/item has its net-worth in a separate field beside the stock-quantity.
To stay in your example: as long as we ship quantity from stock location "a" it's calculated according to the cost of that stock location (and subsequently subtracted from that stock-location).
Meaning: A general total-"average" is calculated during runtime in realtime

Care to share an example?