I thought I'd try a vegetarian version of $40 a day. In general, eating veg is cheaper than eating omnivorously... but I find that at WDW, it can be more expensive due to the limited offerings. Many places only have one option for a vegetarian entree so there's no "choosing something lower-priced."
Yes, there are a lot of pastries -- but I *want* pastries for breakfast on vacation! Also, I eat an app as my meal at a few TSes, but truly, that is how I eat when I eat out. It's generally easier to find (or modify) an appetizer than an entree. I am not much of an alcohol drinker, so I subbed in coffee as my drink indulgence of choice.
Lastly, I'm going in July, so the midday meals are often light by design. This is really pretty representative of how I eat at WDW.
Magic Kingdom
Bfast - Main Street Bakery - Chocolate Croissant $2.79, Milk $2.19 ($5.30)
Lunch - Pecos Bills - Vegetable Burger - served with fries or apple slices $6.79 and water to drink ($7.23)
Snack - Egg roll cart - Vegetable Egg Roll $2.00 ($2.13)
Dinner - Plaza Restaurant - Vegetarian Sandwich, $10.99 with a sundae for dessert, $4.99 and a soda to drink $2.69 ($23.24)
Total: $37.90 (a strawberry yogurt with lunch would make it $40.01, lol)
Animal Kingdom
Pastry $3.00 with Frozen Cappuccino $4.75 ($8.25)
Lunch - Flame Tree BBQ - Fruit Plate $6.99 and water ($7.44)
Snack - Safari Pretzel - Pretzel $3.79 ($4.04) - running total $19.73
Dinner - Rainforest Cafe - Rainforest Pita Quesadillas (app) with no chicken - $9.99 plus soda $3 (estimate) ($16.17)
Total: $35.90
DHS
Breakfast - Starring Rolls - Two mini-pastries plus medium coffee $4.79 ($5.10)
Lunch - 50s Prime Time - Stacked Sandwich (without meat) $11.99 and milkshake $4.99 ($21.14)
Snack - Starring Rolls - White Chocolate Cupcake $3.99 ($4.25)
Dinner - Backlot Express - Grilled Vegetable Sandwich $8.59 with water ($9.15)
Total: $39.64
EPCOT
Breakfast - Sunshine Seasons - Oatmeal w/Brown Sugar $2.79 and Apple Juice $1.69 ($4.77)
Lunch - Le Cellier - Sauteed Potato Gnocchi $18.99 and water ($23.64)
Snack - Boulangerie (France) - Chocolate Croissant $2.85 ($3.04)
Dinner - Kringla (Norway) - Potato, Goat Cheese & Vegetable Tort $5.99 with milk $1.29 ($7.75)
Total: $39.20