At Costco we buy:
Meats (chicken breast & thighs, ground turkey, flank steaks, flap meat which is a smilar cut to a skirt steak, pork chops, and the chicken/apple sausage)
Dairy (milk, butter, eggs). The butter seems to be an amazing deal even when butter is on sale at the grocery store it's still at least $1-2 more than buying it at costco.
snacks like the 100 cal packs and sometimes the Special K fruit bars when they have them.
Medicines: my mom has to take prilosec once a day and they have the kirkland brand for a 45 day supply at $18, can't beat that. We also but kirkland brand aleve, tylenol, and zantac. Pepto Bismol is ridiculously cheap at costco, $7.99 for two huge bottles. I've seen it at target and
walmart at $7.99 for one regular size bottle.
oh and wanted to add... we buy the 6-pack of chicken stock (I use this instead of water in a lot of my cooking) and it's amazingly good. it's the kirkland brand. i buy the regular not organic, I wasn't crazy about it, it seemed to lack flavor.