I too am trying to work out my food budget and expect it to average out at £150 per day for 2 adults and 2 kids (the oldest is 11 so will pay adult prices). But, I am also going to check prices on
www.wdwig.com - great site and really worth looking at the menus - as i haven't been for almost 5 years and the exchange rates were only $1.38 to the £ last time I visited - so everything was so expensive. Also check on Deb's site if you will be eligible for any discount programmes - we are getting an annual pass so that we can get a Disney Dining Experience card and get 20% off many onsite restaurants.
It is difficult to estimate based on what other people spend. There is a similar board on US forum, but many of them don't drink alcohol. I drink wine with meal every night on holiday, so this bumps up the price considerably.
Also, when working out your budget don't forget to add tax - 6 or 7% i think, and tips at 15%. This can soon bump up costs.
Typically, we tend to do one snack type meal per day, one counter service and one sit down meal per day and we work out our budget on those costs. It is impossible to do 3 sit down meals a day, due to portion sizes and the time spent eating would seriously limit your time in the parks.