SQL & PL/SQL :: Stored Procedure With Multiple Table Join With Aggregate Functions

I have 2 tables order_items and items.

Order_items Items
Item_id Item_id
Quantity Price

In normal sql statement: select sum(order_items.quantity*items.price) sales_price
from order_items,items
where order.item_id=items.item_id;

I have to put this logic in either a stored procedure or Function just to calculate sum(order_items.quantity*items.price) and store the aggregated value as Sales_price in DB. Then we have to call this from Informatica Stored procedure Transformation where we will have only one output port as Sales_price,this is to load into summary table. All the aggregate calculations and joining of 2 tables should be done on DB side and only one output should be populated when we execute this procedure.