Calculate the average rate of renting the film that Lucille Tracy got starred.
average rate = divide(sum(rental_rate), count(film_id))
SELECT AVG(T3.rental_rate) FROM actor AS T1 INNER JOIN film_actor AS T2 ON T1.actor_id = T2.actor_id INNER JOIN film AS T3 ON T2.film_id = T3.film_id WHERE T1.first_name = 'LUCILLE' AND T1.last_name = 'TRACY'