There is a poll over on the restaurants board that covers it. Some people spend $100 per adult per day - but they do a lot of sit down at nice places or several character meals, drink alcohol and/or often do three courses.
Other people manage to spend less than $25 per adult per day, but they don't eat sit down - or have a single meal sit down at lunch time when the prices are less expensive, eat breakfast in their rooms, don't drink alcohol, etc.
So the answer is....it really depends on you and how you want to spend your money and how much, how frequently, and how nice you want to eat. Its possible to go with $15 in your pocket (and your own water bottle and an energy bar in your fanny pack) for the day and not starve, and its possible to blow through $100 in a day.
Your best best - as mentioned - is to check out the menus and see for yourself. And - if budget is an issue - check out some of the tips for saving money (like having your sit down meal at lunchtime).
My former budget trips were breakfast in the room (pop tarts, coffee, juice, etc), a small snack (brought in) around 10:00am (like a granola bar). A large late lunch sit down (you don't pay that much more, the food is better, there is more of it, and you get out of the lines at crunch time), then a bedtime snack of microwave popcorn.