We compared prices at Whole Foods to those at Trader Joe's - and the results were surprising

Whole Foods has been desperately trying to shed its "Whole Paycheck" reputation for years as it loses millions of customers to grocery store rivals like Kroger and Trader Joe's.

The news that Amazon is buying Whole Foods for $13.7 billion could make the upscale grocery brand a lot more competitive when it comes to prices.

Amazon has the "financial capacity to price aggressively," Moody's vice president Mickey Chadha wrote in an email.

"Dominant players like Walmart, Kroger, Costco, and Target now have to look over their shoulders at the Amazon train coming down the tracks," Moody's lead retail analyst Charlie O'Shea said.

In April, we visited a Whole Foods and a Trader Joe's to see how their prices on 28 similar items compared. The two stores are just a few blocks away from each other in New York's Union Square.

Overall, our Whole Foods basket was about $30 more expensive than our basket at Trader Joe's, but not everything at Trader Joe's was cheaper. Many prices were actually quite similar at the two stores.

The final bills came to $115.90 at Whole Foods and $85.33 at Trader Joe's, after we accounted for the products' varying package sizes.

Here's how the prices compared: