Database: student_club
Question: Calculate the total average cost that Elijah Allen spent in the events on September and October.
SQL: SELECT AVG(T2.cost) FROM member AS T1 INNER JOIN expense AS T2 ON T1.member_id = T2.link_to_member WHERE T1.last_name = 'Allen' AND T1.first_name = 'Elijah' AND (SUBSTR(T2.expense_date, 6, 2) = '09' OR SUBSTR(T2.expense_date, 6, 2) = '10')