I’m not a Costco member, although keep meaning to join up. Can’t remember what the annual membership is for personal use, but it wasn’t a lot last time I bothered to look (and then never followed up by joining. again).
Have been a few times with a mate who shops there a lot. Always seems to be a lot of good deals on electrical items. There always seemed to be a whole load of random stuff they’d got hold of too, and sold on a “when it’s gone it’s gone basis”, so if you drop in regularly you may come across stuff you need at a good price buy may never see on sale again.
For most other things, you’re saving money by buying in bulk.
Work colleague also found them cheapest for getting new tyres fitted to his car. I didn’t realise they did tyres, but apparently whichever branch he uses does.