We've had tons of Costco threads over the years, but IMO if you shop reasonably regularly at Costco you can easily justify the membership.
Things like OTC medicine, coffee, spices, cheese, most meat (but you must buy large quantities), socks, underwear, men's dress shirts (actually clothes in general), batteries, laundry soap, olive oil, balsamic vinegar, dish soap, paper towels, shop towels, pellets (for the pellet grill).