this movie had some facts - http://www.amazon.com/WALMART-HIGH-COST-PRICE-Format/dp/B000...
Try these facts out as well:
http://web.mit.edu/newsoffice/2006/iap-walmart.html
Quote: "For families with incomes less than $10,000 annually, a super center makes a 30 percent difference in what they can buy. "The marginal utility on the poor is greater," he noted."