I doubt that Walmart's prices can get much lower.  Walmart can sell for low prices by using a very efficient distribution process and selling a high volume of items.  If they charged less, they couldn't pay employees sufficient wages or make an attractive profit for its investors.

I suppose that Sam's Club is the version of Walmart that sells at lower prices (by selling in bulk).  Also, I suppose that Walmart could open a chain of dollar stores -- or have a dollar section in its regular stores -- with only items that cost the company less than a dollar.