I have been a member of Costco for years. DH was working near Watford when they opened their warehouse there and he got an invitation to join, did, and we have been going there ever since.
It isn't that it's cheaper, it's that you can buy in bulk and get a small saving. They do sometimes have excellent offers - some that come to mind was olive oil which was a huge saving and rice (in recent years). They also do own brand stuff (Kirkland) which can be good - their shampoo and conditioner is excellent.
Today I spent £250 at Costco but bought: toilet rolls, kitchen rolls, nuts, shaving foam, deodorant, soap, beef, pork, sushi (treat for DS), San Pell water (treat for me 
, 12 x tins choppped tomatoes, sack of potatoes, rice, salame, 2 x books for DS, 12 x bottles of milk, 3 x packs of lavassa ; fruits: strawberries, raspberries, grapes, 4 x mangoes, 12 x oranges, blueberries, pomegranete, pineapple ... and more stuff I can't remember. The toilet rolls alone will last for 3 months +, the kitchen rolls are enough for the year, the potatoes will last the month +, as will the tins of tomatoes, lavassa etc so, if you were to go every 4-6 weeks your shop would/could be considerably less. Mine was so high because I hadn't been for 2 months prior to Christmas so had run out of lots. Also, it is worth remembering that the packs of grapes, strawberries etc are often bigger than the packs you would get at the supermarket - so you are getting more for your money. They are good for multipacks (cleaning products etc) too, making each individual item cost cheaper than some supermarkets.
Tbh, boffin, that suckling pig looks scary. That itself would not entice me to join. All said, it is great for those bulk buy items but I always shop very consciously there - it is easy to go waaaaaaaaaaaaay over budget.
I have also spent £299 today at John Lewis. I needed a new sewing machine and have been putting it off for ages but finally went in today to get one - it is my Christmas present from DH and DS.