Great advice in all of the posts, and I especially echo the advice about looking at menus from some of the other sites to get a general idea of what kind of food you might want to eat. Of course, it really depends on your eating habits, how tight your budget is, and if you generally bring some food with you or not.
I budget around $120-$130 per day for DBF and myself, but this is because we are completely dependent on buying food for all our meals and we generally have at least one sit down meal per day. Some days we spend more than others, but it usually evens out and all told, the average ends up being $120-$130 per day.
A typical day for the two of us:
Breakfast at food court or a quick breakfast in the parks ($20-$25)
A counter service lunch ($25-$30)
A table service dinner ($75-$100)
For our table service dinners, we usually each order a non-alcoholic beverage, a soup or salad each, and an entree each. Sometimes we also order an appetizer or dessert to share. We might have a snack here and there, but probably not every day.
As I said earlier, we don't eat this way every day and of course the prices can vary, so some days we won't even spend $100, and on other days, we may spend closer to $150. When planning my trip, I usually make a list of the "must eats" and make ADRs whenever possible.