Some people like to spend a lot of money on their Halloween costume. Others like to make them themselves. Finally, you have some people who just want a cheap and simple costume, just to fit to a the Halloween party.
If you are in this last group, here are some nice options from you.
Price: depends on size, the Men MED one is just below $20.
This is another nice costume. Some drawbacks: the mask is not so good and the scythe is not included (you can get a cheap scythe here). For less than $20, this is still a good overall costume.
Price: depends on size, $11.48 for Large
More a T-shirt than a costume but it does the job. Fits Halloween, the Dark knight movies, etc... Another advantage is that you won't have costume problems (something that falls down, movement limitations, ...) during the Halloween party with this costume.
So here were 3 options, for you. If you want to browse more costumes at Amazon, have a look here.