Left outer join not returning all rows in a grouping query
The WHERE o.date ... condition makes the outer join behave like an inner join, cutting out any rows of peoplethat don't have at least one matching row in orders.

You'll have to move the condition about o.date from the WHERE clause to the joining ON:

SELECT p.id, p.name, coalesce(sum(o.price), 0.00) AS total
FROM people p
  LEFT OUTER JOIN orders o
    ON  p.id = o.person_id
    AND o.date BETWEEN date '2014-09-01' AND date '2014-09-30'
GROUP BY p.id;


Test in: SQLFiddle