There have been a couple of studies showing that people who pay with cash tend to buy less than people who pay with plastic. For example, a study at McDonald's showed that the average cash purchase was about $4.50, but the average credit or debit purchase is $7.
To some people, the conclusion is that people with cash on hand are incapable of spending more than what's in their wallet, so they generally are more picky and spend less. Personally, I believe that it's largely poorer people who are paying with cash. There's no good way to know, though.