While I was sitting around rewiring, touching up, and adding arduino control to my pack, my daughter told me she wanted to be my "Ghostbuster Assistant." I kind of hemmed and hawed and told her I didn't think I had the time or money to build one this year, but the idea simmered in the back of my brain for a while and later on that week I realized that not only did I have enough time to do it, but I had almost everything I needed already on hand!
Total cost for this project: $15.88 (under $20!)
Most of the materials were scraps of cardboard and plywood, various doodads and gizmos I had in my shop, and lots of stuff rescued from the recycling bin. The electronics, tape, and hot glue I already had. All I had to buy was one tube of E-6000 epoxy (4.99) and three cans of super cheap flat black spray paint (.97/each). Also, I had to pick up an old backpack from value village (2.99) for the the straps. Oh, and a khaki jacket from goodwill (4.99).
Here's a brief description of the controls:
***UPDATE 5/4/10: If you make your own ghostbusters costume, go ahead and post some pictures in the comments below. I'll send you a patch!***