This may be of interest for anyone using a Sony body. Photo Continental (in Brisbane) have a new Sigma [Sony] 2x EX DG APO teleconverter on their site for $150 (Australian) (
Search Results - Photo Continental) . It says "Limited Stock" and usual price is listed as $379. I know these teleconverters are currently listed at places like B&H for $249, so it is a pretty good price for a piece of kit that is highly regarded. The Sigma site lists the Sigma lenses that the teleconverters are compatible with. However, it may not be quite as restrictive as Sigma suggest. The teleconverters have a slightly protruding front element. Therefore they must be mated with a lens that has a recessed rear element. Some people have got good results with these teleconverters on other makes of lens.
I am about to find out. They also had "limited stock" on the Pentax mount version for $99! I was going to do a general "heads up" on that deal but it disappeared from the listing as soon as I bought it.