It's probably the new Sony models causing this madness.
I got my 100mm f/2 from the used department at adorama.com for $299. Of course, I decided to buy most of my lenses before the KM 5D was shipping (I started buying Minolta AF lenses before I got a KM DSLR to use them with).
This lens was a bit hard to find then (the 85mm f/1.4s are more common). I was looking at an 85mm f/1.4G on Ebay yesterday in Excellent condition for a buy it now price in the $700s. Now, the 100mm f/2 is apparently fetching more.