As has been stated up thread England has more than 80% of the world's chalkstreams with the Test at the top of the tree and the Itchen not far behind. Neither are big rivers so that limits the amount of available rod space before we start. It is the relative rarity that to a certain extent drives the cost of fishing there, whether for trout or grayling. Don't forget too that it is only quite recently that grayling were recognised as a sporting fish in its own right, not just by the estate owners but by a large proportion of anglers too.
OK, you can get good grayling fishing in the Midlands, the north, Wales & parts of Scotland, but they are not chalkstreams and are hardly known about outside of their immediate area. The Test, Itchen and other chalkstreams are known worldwide and there is a queue down the road of people from the UK and the rest of the world who want to fish them for trout, and there's a good few who want their grayling fishing as well. Supply and demand, market forces, call it what you like, but that is what drives the cost and as long as anglers stick to the rules and don't deliberately target the trout then those costs will be reasonable. As soon as anglers start to f*ck about they will only have themselves to blame if the price rises or the fishery stops day tickets altogether.