I believe it ends up being slightly less expensive to live off campus depending on where you live and how often you cook at home. Many students who live off campus but are on campus a lot keep a larger meal plan amount. JMU is rated very high for campus food.. it is good... I will actually east sushi prepared on campus... so, many students choose to eat on campus often even when living off.
Also -- other than the food coop downtown... which is very good, but a little more expensive... and the farmers market when in season... there is not a supermarket within walking distance to the downtown area close to campus. The buses do go to Walmart, Target, and other areas with grocery stores so that is a possibility for shopping trips if a student does not have a car. Also -- many students do have cars, so carpooling is always a possibility.