We found $40 per day per person was a pretty good estimate. If you CS for Breakfast and lunch with a moderate dinner this figure works well. Some friends of ours went using the same figure and they also felt it was a decent estimate.
Example of some of our meals: (We had resort refillable mugs and I took a case of bottled water. We bought the drink straps and each carried our water bottles and used Crystal Light drink packs when we refilled the bottles)
Breakfast at Food Court in resort: French toast & sausage $6, cup of fruit $2-3, Mickey Waffle $5, Deluxe breakfast w/ eggs, biscuit, potatos & sausage $6
Lunch at CS in park: Burger and fries with a ice water(free) $8
Lunch at Food Court in resort: Double Cheeseburger w/ fries $7 or chicken Caesar or taco salad $7
Earl of Sandwich DTD, approx $6 for the sandwich. Chips, salads & drinks extra
Dinner: Food Court $10 - 14
CS in park: Chicken Platter in Tangerine Cafe, Morocco $13, Chicken/Rib platter at Cosmic Rays MK $13
Garden Grill, Epcot: $23 per person (all you care to eat)
Rose & Crown, Epcot: Fish & Chips $14, deserts $4-7
RainForest Cafe - $15-25 for adult entree, drinks in fancy cups $6 & up
Deserts $6-12
Snacks: Frozen Lemonade $4, Turkey legs $5, Churros $2, Rice Crispy Mickeys $3
It is possible to have a lot of variety without breaking the bank.
Happy Planning