Have you read this thread? I can't testify to how well it works but it seems like a homemade carbon scrubber would be the cheapest way to go.

http://boards.cannabis.com/showthread.php?t=41861

I need to figure out some better odor control as well. I will be growing some Ak-47 soon and from what I've read it is one of the smelliest plants around. I plan on trying to build my own carbon scrubber like in the above thread.

Good luck