title: Calculate expanded_price using Django ORM
body: <p>The raw SQL will perform mathematical calculations using calculated field, for example:</p>
  
  <p>The item_price column contains the per unit price for each item in an order. To expand the item price (item price multiplied by quantity ordered), could do the following:</p>
  
  <pre><code>MySQL [distributor]&gt; select prod_id, 
      -&gt; quantity,
      -&gt; item_price,
      -&gt; quantity*item_price as expanded_price
      -&gt; from orderitems
      -&gt; where order_num = 20008;
  +---------+----------+------------+----------------+
  | prod_id | quantity | item_price | expanded_price |
  +---------+----------+------------+----------------+
  | RGAN01  |        5 |       4.99 |          24.95 |
  | BR03    |        5 |      11.99 |          59.95 |
  | BNBG01  |       10 |       3.49 |          34.90 |
  | BNBG02  |       10 |       3.49 |          34.90 |
  | BNBG03  |       10 |       3.49 |          34.90 |
  +---------+----------+------------+----------------+
  5 rows in set (0.027 sec)
  </code></pre>
  
  <p>How could I accomplish this with Django ORM?</p>