Database: movies_4
Question: What is the average revenue of American movies in 2006?
SQL: SELECT AVG(T1.revenue) FROM movie AS T1 INNER JOIN production_COUNTry AS T2 ON T1.movie_id = T2.movie_id INNER JOIN COUNTry AS T3 ON T2.COUNTry_id = T3.COUNTry_id WHERE T3.COUNTry_name = 'United States of America' AND CAST(STRFTIME('%Y', T1.release_date) AS INT) = 2006