Genuinely Frightening
For anyone who does not want an amusing costume: full masks, fake scars and silhouettes that work without a punchline.
Frightening is a question of method, not of budget. A static mask stops working after two seconds of being looked at; the skull mask with a movable jaw does not, and that movement is what you are paying for. A dress already stained and torn looks the same at midnight as at eight.