Without knowing how long you're staying, how many adults, how many children there are in your party, and whether you have pasta taste or steak taste, I can't really give you any budget numbers. But I can give you some tools to figure it out yourself.
The first step I would take is to get yourself a good guide book. My favorite is the
Unofficial Guide to Walt Disney World. Take a look at the descriptions of the restaurants and choose a few that you think look interesting. Then, go to the website allearsnet.com. Click on the search button (located near the top right) and type in the name of a restaurant that looks appealing to you with the word "menu" next to it. (For example, Garden Grill menu, or Brown Derby menu.) You'll be taken to pages with full menus as well as pricing for those menu items. With this information, you can sketch out the budget that will work for your particular situation.
The Disney Moms have answered related questions
here and
here. But these guests' situations may not be similar to yours, so I urge you to work on your own unique budget using the tools above.