haloween party!

ho ho!! i am going to a haloween party this weekend, my first ever! Its called the THRILLER - HALLOWEEN MASQUERADE@ WESTIN ST. FRANCIS
Looking foward to it, it boast that it is "The biggest, most incredible halloween costume party at the largest venue in town- the Westin St. Francis in union square. Over 2000 people expected to come on out at the largest asian-american halloween masquerade in the USA!"
It's going to be awesome!
problem, i have to get a costume, but what costume should i get? and i have only so much money to spend, so a costume that blends in and yet not too pricy at the same time. Should i be a pirate? or a monster? or superhero? or DARTH VADER!!?? going for costume shopping after work, i'll post pics of the party, promise.
http://store.elementsf.com/thhamawestfr.html