Step 1: Get a bed sheet, preferably white.
Step 2: Cut holes in the area where your head may be.
Step 3: Put it on.
Step 4: Go and mug someone who has a really nice costume. (They can't see your face)
Step 5: Use whatever they had.
Side Note: Don't like it? Mug someone else in the same way! The bed sheet saves cash due to inexpensiveness and can be found at home!