Here's my two-penn'orth.
The Nikon 200-400 costs about £4.5k-£5k depending on where and when you buy it. The Canon has the built-in Extender which I can see adding £500 to the price. Plus if it's been treated to the same weight-loss programme as the other Canon super-teles, it will be a lot lighter than the Nikon, and that's worth paying for too.
Bottom line: I reckon it will launch at £8k or more, and in the long run it will *eventually* drift down to about £5.5k-£6k.