We've been costco members for 10 years. Even though there are only 3 of us at ome now, we still save a significant amount of money using costco. We have the Executive Membership, which gives you a percentage back on your purchases at the end of the year. Our winds up refunding a bit more than the cost of our membership.
It works for us because we buy all of our prescriptions at Costco, and the co-pays count towards the rebate. A lot of basics like milk, eggs, and cheese are a lot cheaper. So are packaged, sliced deli meats.
We have a house, so we buy our garden and lawn care products there. Coctco frequently sends you coupon books, and we tend to stock up on things like Lysol wipes, OxyClean, and other things like that, then.
You also can't beat their humungo pumpkin pies for $5.99. Costco also makes the absolute best sheetcakes, too.
Books are always a stellar buy at Costco.
There are lots of things that are good deals, and some things I don't buy because I can get better sales (think paper towels, toilet paper, etc.)
If you really aren't sure, go to the customer service desk and ask for a day pass. You won't be able to buy things at the listed prices, but you can get a good idea of what the prices are. Also, the Costco pharmacy is open to all whether or not you have a Costco card. Their prices are extremely good. Check the website for prices under "pharmacy." While you are waiting for your prescription, check out the store!