Just got the latest Calumet mag with the 300 & 400mm MkII lenses pricing is jaw dropping!
EF 300mm f/2.8 L IS II USM Â£7499
EF 400mm f/2.8 L IS II USM Â£11499
Not many enthusiast photographers going to be buying these, and as for pros well you'd need to making a substantial income to be able to justify either of these.
Those prices seem accurate. Same as B&H.
Canon has already sold a lot of those lens, but of course they are mainly aimed at pros, especially those that shoot sports.
For UK resellers as opposed to grey imports, I would recommend using camerapricebuster.co.uk
300mm f/2.8 mk II - £4999
400mm f/2.8 mk II - £8199
so Calumet appear to be indeed over inflating prices significantly.
If you don't mind the risk of grey import, then I just picked up the 300mm mk II (it's literally in transit) for £4577. If OP wants details look at my other posts or message me otherwise I sound like a sales person