Well, I guess I figured the integrated 1.4x TC added a fair bit to the cost, but perhaps not. I'm also not sure I believe the difference between a 71.4mm aperture (400/5.6) and a 100mm aperture would literally cost thousands of dollars for the front element...were talking a difference of roughly 28mm in size. I'm sure it adds to the cost, but if we take the current 100-400 f/4.5-5.6 which runs about $1800, to the 200-400 f/4 which is supposed to run around $7000...were talking a $5200 difference in price. I don't think the entire difference is allocated to glass...there is the integrated TC, R&D costs to design the thing that need to be recouped, etc.
A normal 1.4x TC costs some $500, and if we assume the built-in TC for the 200-400 is explicitly tuned for that particular lens, it probably cost more than that. Knock off $800-$1000 of the price of the 200-400 for the integrated TC, and we have a price difference of around $4300. I would NOT necessarily expect the kind of IQ from the 200-400 out of a 100-400...I'd expect it to be about as good as or maybe slightly better than the current 100-400, and there wouldn't be an integrated TC.
So, perhaps $3500 is overly hopeful...lets say $4500? Is that any more reasonable, assuming current 100-400 f/4.5-5.6 IQ, with a modern IS system? I'll certainly add that the short end really doesn't matter a wit to me...it could be 200-400, or even 250-400, if that would make it cheaper, as I rarely ever use the 100-200mm range on my current one, as I mostly photograph birds and wildlife with it.
Again, I'm just probing the pluasibility of such a lens, and currently, it seems particularly implausible at around $3500...