It's always good to see a 3 plus year old thread reborn... :-D
For whatever it's worth, I got the cold knife from Kelley Bees as part of a extracting kit whenever I bought my extractor. Figured I'd give it a try on my first extraction and see how it went.
I extracted about 260 pounds with it in August and another 69 pounds in October and couldn't have been more pleased with how it performed. As a solo item, it sells for $17.00 on the Kelley's site...as bailey pointed out, I think the sawing motion is the key with a serrated knife.