We have the trade membership so get 2% back and that pays for the annual renewal. We have been members for about 15 years and every year or so when we do our usual shop from there I write down the costs and then come home and do a comparison with Asda. However, we now shop at Aldi so some things may work out much cheaper from there. It depends if you buy branded products too.
Lots of things are cheaper, some are just incredible quality in terms of taste or product. We have bin liners from there to fit our large brabantia bin and never had one rip, ever. Tinfoil and cling film, also really good quality but come in a massive roll. Kitchen roll is taller and better but again comes in a massive pack.
You need space to store it, ie tinned tomatoes comes in a pack of 12, toilet roll is 40 rolls, tissues are a box of 8 or 10. It all takes space. I have an American fridge freezer in the kitchen but I also have a large pantry with a larder freezer for the mince 2.5kg packs (roll it flat and vacuum pack it) chicken breasts, divide, vacuum and seal, again this a pack of around 11 or 12, frozen salmon 7 pre-frozen fillets which are much bigger than the ones in the supermarket and defrost in under 30 minutes.
If you can just go with someone for a visit then do that first rather than splash out. Dh used to go every week on the way home from work as it was on route so even our milk came from there. Yes that is cheaper. However now he works from home so it would mean driving there so not as cost effective with the cost of petrol so we go every 2-3 weeks.