One of the most memorable "villains" in the Harry Potter series, Professor Snape is a great Halloween costume. It's also a fairly cheap costume to put together yourself. This is a page about making a Professor Snape costume.
Making a Snape costume for Halloween this year is simple and fairly inexpensive. There are instructions to make a cape and most of the other clothes may be in your closet or are easily obtainable at the thrift store.