Actually, supermarkets are not cheaper - not for the household goods and food we buy. I did a very careful analysis, and we easily cover the cost of membership, fuel and more.
We buy large packs of loo rolls, tissues, kitchen rolls (but not blue rool, Nisbets are cheaper). Plus catering packs of meat (mince, chicken, bacon, etc.) which we split up and freeze. Then there is the cat food, soup, Marmite, olive oil, etc.
I have to say that on our second visit we bought a load of storage shelving...
Typical spend for us is £500-£600 a visit - perhaps five times a year.