Amazon have these Halo 3 Covenant Energy Sword for $44.99 right now. These can inflict some serious damage if you’re a sick individual. But they are a perfect set to go with your $700 Halo 3 Master Chief costume. I remember Link posting another one of these some time back and they go for $300 then (it might have been custom made).
edit: fixed the link.
GamesOgre posted this on the comments: Halo Reach Inflatable Sword. According to PapaStu, these were given away at Best Buys here in the U.S. It’s at $20 right now.. thats right, $20 for the inflatable sword! There’s another one listed but it’s $60. You might as well buy the real thing then for $45.