SQL & PL/SQL

Hello,

I am trying to calculate the inventory value based on the available stock .

For example

Now calculating the inventory balance

 select nvl(p.branch_id,s.branch_id)Branch_id,nvl(p.prod_id,s.prod_id) product_id,
       nvl(p.quantity,0) - nvl(s.quantity_sold,0) Stock_Quantity      
  from (
  select branch_id,prod_id,sum(quantity) quantity
          from costs
         group by prod_id,branch_id
       ) p
       full outer join
       (select Branch_id,prod_id,
               sum(quantity_sold) quantity_sold
from sales
         group by branch_id,prod_id
       ) s
    on p.prod_id = s.prod_id
    and p.branch_id=s.branch_id;

will give me

Total inventory balance = 4

But calculating the inventory value seems to be troublesome for me ...

1 piece at $1100 and other 3 at $900 = $3800.

Thank you very much for any helps