Take a look at the salt shelves in any of the big supermarkets. You'll invariably find that the Saxa salt drums are fairly full of stock whereas the supermarket own brand ones are depleted.
Currently, Tesco sell a 750g drum of Saxa salt for £0.90. Their own brand salt container of the same weight sells at £0.38. The curious that look at the ingredients in both cannot see any difference in the ingredients. Therefore,they cannot justify in their mind the price disparity. The end result is that the own-brand products sells in vastly greater quantities.