I try to use costco to get certain basics plus regular/reliable cost 'treats'.
Some things are the same cost as elsewhere, only come in bigger quantities so save me time to get at costco, others are cheaper. Price per item is helpful in figuring it out. Only buying things that you will use up makes a difference too.
I get: frozen pizzas, eggs (when not from our farmers), yogurt, spinach, berries/grapes/kiwi/banana (only one of those per trip), flour, butter, , peanut butter, jam, tortillas, tortilla chips regularly.
Sometimes we get veg burgers or sausage, sliced cheese, frozen burritos, coffee, vanilla, sugar, rice, olive oil, tunafish.
For treats I might get croissants or muffins once every few months (and we freeze half or so of them). (I'll add that I overdid this for awhile, and am now a little burnt out on these, but since I find them pretty cost effective and they're the only 'treats' I really allow within grocery shopping it's ended up balancing out nicely).
I find the biggest money savers to be the pizza, yogurt, flour, spinach, tortillas & chips, and eggs when we get them - similar quality or quantity of those elsewhere would cost a lot more for us. We take out cash only for costco and try to use that to help limit buying too much (which helps a lot, but so does me going there without dh. . . )