You can not be exact here but the average over 10 products for 5 KG is £10.65 But i can get it for about £8 on eBay (if you use eBay). This is right for the 11/10/10
Unless you need it in extremely high purity for some reason, it's pretty cheap, and you can check on it yourself by going to the grocery store and seeing how much table salt is selling for.