Among all the menu pages with the appearance of the dish "Clear green turtle", how many of them have the dish at a stable price?
Clear green turtle is a name of dish; stable price refers to highest_price is null;
SELECT SUM(CASE WHEN T1.name = 'Clear green turtle' THEN 1 ELSE 0 END) FROM Dish AS T1 INNER JOIN MenuItem AS T2 ON T1.id = T2.dish_id WHERE T1.highest_price IS NULL