What is the average amount paid by students in a position other than a member?
position other than a member refers to position ! = 'Member'; average amount paid = DIVIDE( SUM(cost), COUNT(event_id))
SELECT AVG(T1.cost) FROM expense AS T1 INNER JOIN member as T2 ON T1.link_to_member = T2.member_id WHERE T2.position != 'Member'