So basically this is a 200-560mm f/4-5.6 lens? Now, due to the IS, if it is a highly portable (hand-holdable) lens ... great for sport and stalking game. If not, still, what's the point?Well, a 400mm f/4 lens will cost you $6000+ from Canon. A 500 or 600mm lens from Canon will cost you $10k (though, they will be f/4, not f/5.6). A 200mm lens is another $750, and a 300mm f/4 is another $1000. So, in pure focal length you are getting a 50% discount. And of course you'd have to change between all of those. A 100-400 would presumably be an alternative, but, its an older design and you need to add a teleconverter to get to 560.
I dont see the discount - I see a premium.
By your numbers,
200mm f/4 = $750
300mm f/4 = $1000
400mm f/4 = $6000
TC 1.4 III = $500
You get the 500/600 f/5.6 via the 1.4 that is also part of the 200-400 mechanism.
(edit - assuming the 1.4 works on a 400 f/4.0)
I total roughly $8250 done via components versus $10000+ as a single assembly.
This is by no means a bad deal. For the 25% premium you get an all-in-one that saves you swapping time or additional bodies so as not to miss the shot. But still its all f/4.0+.