I've got a D700 for FX work- non telephoto, and I can't see me ever needing to change it. If I didn't have it then I'd get a D800 for FX, but really the 700 does all that I ask of it. An 800 would just be equipment fetish.
My DX cam on the other hand (a D300s) does the bulk of my photos and virtually ALL of the difficult stuff.
When I go to an airfield or an airshow, it's the DX body all the time. So a D400 with better AF, better crop-in-ability, maybe a bit better dynamic range (not even sure what that will mean but can only be good)and robust body is much more useful to me than trying to improve on my already lovely FX body.
Also, when I can only take one body- it's always the 700 that gets left at home.
Frankly, they can charge what they like. I'll be getting one.