Thift stores always have pieces that you can put together to get a scary costume on the cheap. It usually depends on how much time you have to look and what your vision of scary is. But one thing for sure, it won't look store-bought and you won't show up with the same costume someone else has. Savers has cheap costumes that are new, too.
I aways go to Goodwill first, believe it or not I find all kinds of stuff their for really cheap. I bought a cheap wedding gown & went as a dead bride the first year I was married, 25 years ago. I also found several scary costumes for my daughters there too, one went as a dead cheerleader & one a Vampire all from Goodwill. witchy46
It never hurts to buy a nice mask and then cheap clothes from a thrift store. I make nice leatherface masks and then buy really cheap crappy clothes at liberty thrift store and wear them down and dye them.