I agree with other posters who have suggested having a look at the menus of the restaurants that interest you and adding up approxiamately what you would spend if paying out of pocket.
We did the dining plan in 2005 and, no doubt, it worked out to be great value for our family. The 2 "problems" we noticed at that time were: 1) It was too much food and we would end up over eating. 2) There were times when it was a bit restrictive and/or inconsistant. Example, I am a light breakfast eater and would usually just have a bagel from the food court. Sometimes the cashier would charge me a full counter service meal for just a bagel. Other times a different cashier would allow me to get 2 bagels for a counter service and 1 time I was only charged a snack credit.
We are going back in 2008 and will not be getting the dining reason for 2 reasons: 1) Now that you have to pay extra for the tip and the appetizer is not included it does not seem like that great of value. 2) Our oldest child recently turned 10 and, while the children's price seemed great value, the thought of paying $40 a day for her sounds crazy to me.