There's a rear light on Deals Extreme that is less than $4 shipped. Someone wrote a good review on it. It's not as bright at the PB Superflash, but there's a wider angle it lights up, making it more visible from the side. The battery life is just as good as a SF. For $4 it's hard to beat and I bought two.

You would have to make something to get it to mount on a rack though. It just comes with a mount meant for seatposts.