data pulled from sub form not working

[dm117]

i calculated the quantity * product price in my sub form with ---> =Sum([Quantity]*[Product Price])
then i displayed it in my main form with ---> =[Orderproductsubform].[Form]![Totalcost]
the total is then shown in the order cost field in my main form but when i click save, the order cost field does not save any data

any help will be apperciated

[June7]

Should not save aggregate data, calculate when needed.

Requires code to save calculated data.