I cannot believe the price of butter! £2.50 for a standard and £3 for a "posh" brand! Completely shocking! Like Dragn, I would struggle to tell the difference between them, but I think butter is a good product with no nasty additional ingredients - and I like to buy British - so that is what influences me on which one to buy. (ie not cost) A pack will last a good while, so I don't mind.
I have mentioned before that it is possible to buy cheaper versions of staples like tinned tomatoes and beans if you go to the "foods of the world" aisles, but even there the prices have skyrocketed! Not so long ago, I could get a tin of chopped tinned tomatoes for 30p - now 45p. (Still cheaper than the 60 - 75p for other branded versions though.)