I was at TRU earlier today, browsing the Gamecube section, and as I passed the space where they usually had a few copies of Mario Mix, I noticed it was empty. I kept passing until I did a double take on the price tag on the shelf... $9.90.
Now, more than likey, any copies that have been clearanced have been snatched up by eBay hawks or TRU employees by now, but it's certainly worth a shot at your local store.