Hi Kelly,
Love that you're planning ahead so you'll know how to budget for your trip! It will depend on where you eat. At Cosmic Ray's located in Tomorrowland, you can get a burger and fries for around $6.60 up to 1/2 chicken and BBQ rib combo plate for around $13.99. Soft drinks can run around $2.50 for a large (a large is the best value for soft drinks). At the Columbia Harbor House in Liberty Square, you can get a bowl of chili or soup for around $4.50. A chicken nugget and fish combo runs around $8.20 (drinks are not included in combo prices).
You can check out the menus and prices for all restaurants (table and counter service) at
www.Allears.net . Its an unofficial Disney website and their listings may not all be up to date and are not guaranteed. I have found them to be very reliable and they do a great job of gathering info and listing it in a central location. Please remember that menu prices and selections are always subject to change but you can at least get an idea of what you're looking at as far as prices go for menu options. To find the menus, click on the "Dining" option on the top of their homepage. Under "What's on the menu", you'll see a link for "current menus and the prices".
If you're looking for a light dinner and want to save money, you many consider splitting an entree or sharing a large drink. A lot of the menu items are listed as combos (i.e., hotdog and fries or hamburger and fries). You could ask for just the hamburger or hotdog without the fries and save a little money.
Wishing you many magical memories :-) Allison