Total cost was less then $50 AU. And a few hours of work.
Step 1: Inspiration
Well, I played Monkey Island a lot when I was younger, plus it was away from the norm. We where going to a pirate party and we didn't want to dress up as the usual pirate with an eye patch.
Guybrush's attire changes throughout the Monkey Island series, the costumes we based these on were the characters form Curse of Monkey Island (Monkey Island 3). Two screen captures of charcters are seen below (source Guybrush Threepwood and Elaine Marley: A Visual History)
As a second source we also used some good deviantART fan art, Elaine Marley, Guybrush Threepwood and together.