Other users have reported being able to get the application for free (as they should be able to) by changing their system language to English, restarting the computer, and then trying to update. If you are not using English currently, try it. Of course you will want to switch back afterwards.
Best of luck.
I'm having the same problem. My system is already set to English so that's not the problem. I have iMovie '09 installed on my systems and all the iLife applications have upgraded but iMovie is still showing as a $14.99 download. Any idea how to get the App Store to recognize that I have iMovie already installed? From my understanding, iMovie '09 qualifies for a free upgrade.