We are just a couple...living on Hawaii Island. We buy ~90% of our groceries at COSTCO. We have the Executive membership, and receive 1-4% back on purchases. We just received our annual refund check, for $436! In addition to the refund check, we save about $0.90 per gallon on gasoline. This year, we saved $500 on a washer/dryer, $1000 on a garage wall system, and $100 on a Staub pot. It's well-worth it!
We used to also belong to Sam's Club in Honolulu, where they had better quality produce than COSTCO. They also had smaller sizes of items, so the membership there was also worth it!
Supposedly, Costco Honolulu (and maybe Sams AND Home Depot and a couple of the other biggies) are THE busiest of their individual stores in the USA. IOW "our" Costco (not the very busiest) often has no parking spaces. The busy one "downtown" (the one with the gas station with 50 pumps) is THE busiest in the nation (we hear.) I believe it.
Imagine cruising around a big city block at Sams/Walmart - just to get into the 4 story, city block parking structure at Christmas time.
Costco and Sams "revolutionized" shopping in Hawaii. These two stores actually make it possible for some folks to remain on Island. We easily spend $5K/year at Costco. Not everything is a bargain but you learn quickly what is.
Some of our grocery stores will occasionally have loss-leaders that draw us in, but otherwise, virtually all our shopping is at Costco (and to a much lesser extent) Sams/Walmart - it's much farther away and doesn't have gas in Honolulu.
Agree with HI Bill on gas, though when fuel prices are stable for a year or so (when was the last time?) we only save $.30 to $.40 in Honolulu. But typically, we save $.50 to $1/gallon. In pre-Covid days, that paid for our membership.
Some things about Costco (and to lesser extent Sams) that irritate: They have NO cereal (except maybe Cheerios) without added sugar. ONE time they had huge boxes of my favorite cereal (Grape Nuts). I bought two boxes (80% savings over any grocery store in town). Next time I went, they had none and had stopped carrying it. The story I finally got was "The cereal company could not supply them with enough to meet demand and still supply "real" grocery stores." No idea if this was true, but they do not carry Grape Nuts.
Now, multiply that by many other items. I have a saying about Costco: If I like a product, Costco will stop carrying in! Still it (and Sams) are the only game in town. SWAG is Costco saves us 40 to 50% on those items we buy there. To some extent, it's a "Hawaii thing" I'm sure. On the mainland, Costco is too far away when we are back there and Sams is pretty much where we buy our gas on the mainland. YMMV