The street price of the Nikon version is Â£5000 the RRP is higher at Â£6200. Generally Canons lenses are a bit cheaper than Nikons for the same focal length, but this one has the 1.4 TC built in, so for it to compete with Nikon I'd suggest a street price somewhere around the same.
The bigger issue is demand, and I wonder how many will be sold, as I've never seen the Nikon version, Nikon users speak of it in hushed tones as if it's some mythical beast, most outlets don't even appear to stock it.
When I want these kinds of focal lengths I have the Sigma 120 - 300mm f/2.8, that's now available with OS and it has a good reputation. It's a full stop faster than this Canon through half its range, and with a 1.4x tc it reaches 420mm at the same aperture, with a 2x it gives 240 - 600mm f/5.6 All this is (including the TCs )available for Â£2100 which is about a third of what is being projected for the Canon.