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.