If someone said you could earn $250 by spending $130, would you do it? What about spending $65? What if it’s an annual fee but the savings are unlimited?

This is the situation I found myself in after signing up for a Costco membership last week. I knew my savings could be significant if I shifted some of my shopping to Costco, but I was stunned by how much I saved in a single visit.

Before making my first visit to Costco, I made an extensive shopping list. My husband and I went through our usual purchases and thought about what might be worth buying from Costco instead. We are a family of two, so we know that we cannot consume large volumes of food with a short shelf life. But purchasing pantry staples and non-perishable items is exactly our job.

We knew in advance what we were looking for by going through a list. This saved us from wandering up and down the halls, getting distracted by all the deals. If we had walked in empty-handed, we would have likely walked out with items we didn’t need or weren’t actually priced lower than other stores.

Are you a Costco member looking to save even more? Check out the best credit cards to maximize your spending at Costco.

Save $250 on just one Costco trip

When we got home from the warehouse, my husband did a little spreadsheet magic to compare our expenses that day to what an equal amount of the same items would cost us at the regular grocery store. And we were amazed by the results.

We spent about $280 on our first Costco trip. But based on the prices per unit of each item, recorded $250 compared to what we would have spent otherwise. This means almost 50% savings! And because we stuck to our list, all of these savings were on items we regularly buy and would still buy.

We are $120 ahead of the membership fee we paid in one visit. And we still have 12 more months of spending and saving to look forward to.

We were also able to earn even more by using our rewards credit card for purchases. Take a look at this curated list of the best credit cards to use at Costco.

How to decide if Costco is valuable to you?

The best way to figure out whether becoming a Costco member is worth it is to do a little homework first. Always think about the products you buy, whether it’s oatmeal, olive oil, or printer paper for your office. Take note of the price you typically pay for these items, and then look at Costco’s website to find the price of a similar item.

Next, you’ll want to do some math to find the per unit prices of the two. Your regular cereal might cost $6 at the grocery store or $10 at Costco. But if the cereal is sold in a 20-ounce box at the grocery store and a 50-ounce box at Costco, you can’t make a direct head-to-head comparison of prices.

The price per unit will give you a more accurate comparison so you can see if buying that item from Costco is really worth it. In this example, your regular cereal is $0.30 per ounce, while Costco’s is $0.20 per ounce—a much better deal.

Gold Star membership costs $65 per year, while Executive membership, which offers 2% cash back on all eligible Costco purchases, costs $130 per year. However, Costco will let you downgrade or even cancel your membership at any time if you think your savings aren’t significant enough.

If after doing your research you think Costco could save you some money, give it a shot. You can save much more than you think.