I use this tool, and then add about $50 per person for extra snacks, drinks. I'm not sure how accurate or up to date it is, it has Jungle Skipper Canteen, but not Tiffins. So I use it as a starting point and add extra.
http://www.distripplanner.com/
As far as the OP question, we don't budget but we are naturally cheap. Using the site LadyRayado posted, for 2 adults for 11 nights, it would cost 1,022.77. That would be for 3 buffet breakfast and 2 buffet dinners, 18 CS and about 2 snacks a day. But that would be more than we would spend. First we have some discounts with DVC, AP etc. At CS, we share a large drink, so that a few more dollars off per day. We also instead of eating 4 CS per day, we only get 3, sharing one at lunch. So our typical day would be eat breakfast in room, share CS for lunch, have our own CS at dinner and 2 snacks per day. If we are having a TS that day, our only other purchase would be a shared CS and snacks. So our true cost would be more like $875