Here's how much it costs to buy organics at Aldi, Trader Joe's, Whole Foods and Walmart

Almost half of Americans buy organic food at least some of the time, according to a recent poll by organic produce company Earthbound Farm. And millennials are particularly enthusiastic, with one in five saying they purchase organic products all the time, the poll finds.

But organic food can be a lot pricier than the conventional kind. Last year, shoppers paid roughly 7.5 percent more for organic items, according to Nielsen research. For example, the company found, organic milk sells for $4.76 on average, almost double the average cost of regular milk at $2.59.

So which stores generally have the lowest prices for organic food? CNBC Make It worked with grocery price comparison app Basket to determine the average national cost of organics at Aldi, Trader Joe's, Walmart and Whole Foods. We compared store brands wherever possible.

After crunching the numbers on 19 everyday grocery items — ranging from bananas and canned tomato sauce to white bread and peanut butter — Aldi came out ahead in more categories where all four retailers sold a similar product.

Aldi offered store-brand organic versions of 15 of the 19 items we sought, and its prices were competitive. Its whole frozen strawberries, for example, cost $.18 an ounce, less than frozen strawberries at the other stores.

Price, of course, is not the only relevant factor: Quality matters as well. That's why many shoppers are loyal to Trader Joe's, for example, or to Whole Foods, which has operated as a "Certified Organic" retailer since 2003. The chain is known for its organic, humanely raised meats, and its 365 Everyday Value brand dairy items use milk from animals that are not given synthetic growth hormones, a company spokesperson notes.

Whether you're shopping at your local grocery store or a big box store, you're probably looking for value — good quality at a good price, says John Karolefski, grocery store analyst and editor of Grocery Stories. If you shop at Aldi, you know the grocery chain often delivers.

"Aldi, happily, has a lot of good quality, good-tasting products at good prices," Karolefski tells CNBC Make It. "It's one of the reasons they've been so successful in the U.S."

"The best place you can buy organics, and they're continuing to roll out even more, is probably Aldi," agrees Phil Lempert, food industry analyst and editor of SupermarketGuru.

Part of the secret to the store's low prices is that the vast majority of their products are private label, so you're not paying for the marketing and advertising that many brands must use to attract customers.

It's a strategy similar to the one used by fan-favorite Trader Joe's, which also offers a lot of good prices on the organic products we compared. It's worth noting that Aldi U.S. and Trader Joe's are independently operated companies with distinct but once-related corporate parents. It could be said that Aldi and Trader Joe's are "estranged cousins."

Aldi's SimplyNature brand offers shoppers great value on organic products. Elsewhere, those "can be pricey, but you can get them for a good value at Aldi," Karolefski says.

In addition to being organic, frugal shopping expert Lauren Greutman reports, the SimplyNature products do not contain over 125 ingredients that experts have deemed questionable, including artificial flavors, high fructose corn syrup, trans fatty acids, nitrates and propylene glycol.

Yet Aldi's selection of organic products, while growing, remains somewhat limited. So if you're shopping at this grocery chain, you may need to visit another store to get everything on your shopping list.

Correction: We have updated the prices on the Whole Foods organic 365 line of black beans, olive oil and chicken after discovering internal mislabeling errors by data provider Basket.

Don't miss:

• 5 foods you should always buy at Aldi
• 8 foods you should always buy at Trader Joe's
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