What is the average replacement cost of the films under the category of "Horror"?
"Horror" is the name of category; average replacement cost = Divide (Sum(replacement_cost), Count(film_id where name = Horror))
SELECT AVG(T3.replacement_cost) FROM film_category AS T1 INNER JOIN category AS T2 ON T1.category_id = T2.category_id INNER JOIN film AS T3 ON T1.film_id = T3.film_id WHERE T2.name = 'Horror'