An Optimus Prime costume I made in 5 days for about $20. Mostly made from lots of cardboard, cereal boxes, velcro, hot glue and superglue, among other things.
I took it out to a midnight showing of Revenge of the Fallen. That was a great experience; people were constantly crowding and wanting to take pictures with me. This video
stands as a testimony against the false stereotype that nerds can't get chicks. :)