you're really not comparing this to the 200-400/4 are you? o.O
Why not? It's only one stop of difference. The 100-400 has the 100-200 range while the 200-400 has the built-in TC. I'd bet the optics are similar and that it would be really hard to tell the difference between them at the same focal length.
I'd say "it's a WHOLE stop of difference". Night and day in those focal lengths. Compare 300/2.8 and 300/4 or 200/2 and 200/2.8 - there is also "only" one stop difference, yet there is quite surprising PRICE difference
Yes, and that's one reason I find Canon superteles so hugely overpriced.
Compare a 70-200/4L IS and a 70-200/2.8L IS and you'll see a difference that I think is much more justified than the difference between a 300/4L IS and a 300/2.8L IS or a 100-400L IS and a 200-400/4L IS.
A lot of that price difference has to do with the size of the optics. It costs much more than 2X the $ to make a lens that is 2x the diameter. You have a much greater volume of high quality lens glass and a much longer time of grinding it to tolerance.
The rule-of-thumb is cubic with diameter.
So, a 300/2.8 is just short of 3 times as expensive as a 300/4. But that's just for the front elements. The rear elements and the rest of the lens are close to the same.
So, I think the 300/2.8 should be about 1.5x-2x as expensive as a similarly high-quality 300/4 or 200/2.8 (about the same size). And that's just about what the old one was. When I was considering it, the 70-200/2.8L IS was $1,700 and the 300/2.8L IS was $3,600. Now, Canon is off in la-la-land.