Our econo-plan:
1) Breakfast in the room (cereal, Poptarts, etc) $0
2) Snack we brought in around 10 or 11 $0
3) Late lunch at a sit down restaurant in the park. Lunch is cheaper than dinner but you still get the great food and experience. This will cost approx $15 per person. Even better is to eat at somewhere like Tusker House where you can get a "real" meal as a counter service. There is will be around $10 per person. If you want to really splurge, choose one of the buffets - Garden Grill is $21 per person for all you can eat.
4) Late afternoon snack in the park - Dole Whip, Mickey Bar, etc. This is less than $5 per person
5) Dinner offsite - either at a restaurant outside, which will be the same price or less than what you pay at home to eat out - or cheese and crackers in the room.
So going economy, it can be done for $50-$75 a day and not feel deprived.
Another hint... buy some of the Crystal Light singles to flavor the water when you refill your bottles. It will make it taste more palatable. Did that last week and were very happy with the results.
If you can get your guys to drink water, instead of ordering a beverage, you'll save $10 per meal (except at buffets where drinks are included).
I don't advocate sharing drinks among all the family - not drinking out of the same cup at least. Nothing is more miseerable than coming home with EVERYONE sick.
And, skip dessert after meals. Snacks in the park are cheaper and, in my opinion, better.
Also avoid the "container" charges - blinky light cups, cups that look like Mickey Mouse, etc. If you want a souvenir, buy one, but a fancy cup may not be the best choice.