Our family of 3 typically has a light breakfast ( muffin and beverage ), a counter service lunch ( hamburger, fries , drink ), a mid afternoon snack, and a nice full service dinner, all on Disney properties.
On our trip last year we averaged $150 a day and at times I felt we were making value choices to save a few bucks.
This year we added the Disney Dining Plan to our Magic Your Way reservations and paying $89 a day. ( 2 adults x $39, 1 9 yr.old child x $11 )
Disney advertising states you'll save about 35% and by my calculations they hit the mark just about dead on.
If your family of 4 tends to do well with fast food fare and light breakfasts, you'll probably average about $100 a day out of pocket. Add a full service meal to the mix and four adults can easily add another $100 to $150 to that total for the day.
If you frequent off site restaurants like the many you can find at Crossroads Shopping Center you can knock a substantial amount off that total. Be aware though that most of the fast food places near Disney property like Mickey D's don't carry the dollar menu items. And items like a Big Mac that you get for $2.29 at home will be more like $3.49 in the Disney area. Even snacks like packs of crackers that go for $.50 at home will be a dollar at Disney, and that $.50 cent Coke ? Forget it !!
A few years back we did a bit of dining off property at places like Cracker Barrell, Longhorns, McDonalds. Saved money but wasted time driving everywhere when we could have been in the parks, had a meal and right back to the action.
Here's a complete listing of Disney restaurant menus and prices
http://www.wdwinfo.com/wdwinfo/dining/disneydining.cfm