Now that I've purchased a welder, I have a lot more need for a wire-wheel to clean things up. So, here's how I built mine very cheaply.
Motor: 1/4 HP electric motor from a garage sale for $5.
Arbor: about $3 at a hardware store.
Wire wheel: I don't remember the cost, but if it was over $10, I'd be shocked.
Base: Made from a 20-inch disk off a plow. $0
Vertical shaft: I bought a bunch of 1/4 inch thick square tubing from a machine that had been cut apart at work. Not sure how much I got, but the whole shooting match only cost me $1. This was a small piece of it.
In the end, I have a very functional little portable unit, and it'll hold me for a few years, anyway.