A survey commissioned by Consumer Reports has revealed that Costco Wholesale is the cheapest retailer to buy groceries in the country, outperforming popular brands such as Walmart. The survey was conducted by Strategic Resource Group (SRG), a consultancy based in New York. In the Boston metro area, Costco's grocery prices held a 37 percent cheaper margin over Walmart. Other popular retailers included BJ's Wholesales and Aldi. The study also revealed that most Americans buy their groceries from at least two retailers a week.