I just recently tried to figure this out for my wife and I. What I did was make a spreadsheet, using google sheets, and priced out everything that we would spend if we did not have a budget. I did this to determine if getting the dining plan was worth it or not. We also usually had the dining plan for free, so I really wanted to see how much money we would spend. We usually don't eat breakfast there (have cereal/fruit/yogurt in the room) Here take a look:
https://docs.google.com/spreadsheets/d/1N-2i-ARxNkFXZUMf0acqUIlIP4kr9aEojE9JTTMyPVY/edit?usp=sharing
I tried to enter the cost of the entres that we would purchase if we had no budget to get a true idea of what it would cost if we got exactly what we wanted. I got the prices off of various sites that show the menus. Also, I know that our choices may change on the day we get there, but it still gave us a good idea. For the drink, dessert, and snack columns, I took added the average cost for those items. My wife and I usually will not purchase a drink with our meals (we drink mostly water) and won't always get a dessert. So I did this to see what would it cost if we did it every time.
Because we will probably some days get a snack, dessert, or drinks with meals, using this spreadsheet I am budgeting around $800 for us in food.
Anyways that was my method. It is hard to say what would be someones average cost, without knowing their exact eating habits and likes. Hopefully this spreadsheet will help you a little bit