I assume it's just that you often can find stuff cheaper - if you happen to find a good half price/2-for-3/etc. deal on branded stuff at a supermarket then Cost

I assume it's just that you often can find stuff cheaper - if you happen to find a good half price/2-for-3/etc. deal on branded stuff at a supermarket then Costco will be more expensive. The quality of their own brand stuff is good, though, and the prices are reliably at least fair and usually better. Their petrol station used to be reliably cheaper than everywhere else near me, but they changed supplier and now it's the same price as Tesco. So I go to Tesco, as it's 99 RON and I get clubcard points.