To justify its existens it has to be sufficiently better than the 600 f4L IS II w. 1.4xIII extender. That requires a hell of a lens and it will most certainly be expensive. I can't see why Canon shouldn't try the same pricelevel as Nikon, or maybe even a bit above. If it sell, they have excellent margins. If not, they can reduce the price and still get a healthy return on investment.
I guess it all depends on the supply demand ratio. How many do you sell when it is US$ 21,000+ and how many do you sell at US$ 17,000 etc.
The Canon lens today is US$ 13,289. Would they really be able to increase the price by over US$ 5,000 or more?
How did the prices change when the 300, 400 and 600 Mark IIs were launched? How much more were they when they came out compared to the Mark Is?