Realistically I think we're looking at $7500 to $9000 for this+1,000
DO lenses are more expensive because of the compact and light weight design. I wonder how light this lens will be? I hope it will be at 1+kg. At that weight I would be inclined to take a long hard look at the lens.
With the fluorite elements added in, I would expect a 400/4 DO to be VERY light weight. Especially if it also uses the new barrel materials found on the 300/2.8 II and its friends.
A lot of the price of these things has to do with the materials costs. It is far from cheap to grow flawless fluorite crystals, or properly etch DO glass elements, or even produce high quality optical glass. The first generation DO lenses lacked IQ a bit as they used a diffraction grating approach. I think new designs have moved towards particle dispersion in optical glass, which should offer significantly superior IQ.
I think the DO game will really get interesting when Canon ANNOUNCES 500, 600, and 800mm DO lenses at half the weight and shorter physical lengths than the current designs. They will probably cost the same, will probably not be quite as good as the non-DO but slightly better than the old generation versions of those lenses. The last generation of Canon supertelephoto lenses offered some of the best IQ you could get your hands on. That kind of IQ in a smaller, lighter, more manageable package that could easily be hand-held for hours...now THAT would be something very interesting.