eBay occasionally...
Compusa recently shut down a store near by my house in which everything was at least 15% off, that helped a little.
I picked up one this week for 45 shipped, used-still waiting on it...but if you don't mind used check the marketplace...
These things aren't very discounted new because they are still the top of the line, and Apple keeps prices firm on their products....
EDIT: some people might be interested in your current wireless mouse, many people have reservations about the BT MM and the MM in particular, which is why I'm hanging on to mine.