Not everything at Costco is a win price-wise, but if you do even a small amount of research you can find a number of items that are huge wins.
A pint of pure vanilla extract for $9.99. Dog treats at about half the price of pet smart or online stores. Kirkland brand Flonase is about 75% less than you can get at a pharmacy. Kirkland brand Claritin is ridiculously cheap, 365 tablets for under $20.
You very rapidly get back your yearly fee.