I usually budget $40 per day for myself, but it can vary wildly depending on whether I eat a table-service meal or counter service all day.
Most counter service meals will cost you $10-$15, but table service has a much wider range of prices and is harder to predict. Most of the buffets and family-style places are between $25 and $30.
A better way to go about this is to look over the
menus on AllEarsNet and pick out your favorite table-service places, made some reservations, and figure up the prices. After that, add money to the budget for counter service places.
NOTE: Table service restaurants at WDW book up quickly, sometimes as soon as the 180-day booking window opens, so reservations are a MUST even in the off-seasons. Walk-ups are not even accepted at some restaurants, and wait times for those that do accept walk-ups can be 60-120 minutes during the busy dinner rush.
Also note that the dinner rush at WDW runs from about 6pm till almost 9pm, partly because people keep later hours in the parks, and partly because of people coming from different time zones around the world being used to eating at, say, 7pm on the west coast, which is 10pm on the east coast. Eating early, between 4:30 and 6pm, is the easiest way to get into a restaurant with no ressies.