We always ate the free continental breakfast at our hotel. We would grab an apple for a morning snack, or have a granola bar.
For lunch we ate at the park.
Supper was sometimes at the park, or somewhere along Harbor Blvd.
There is a few places where we got an individual pizza with a garden salad for around $5, and the kids could share that. Or we would buy 3 orders of that for the four of us. We also found burgers to be around $5.00. Kids meals were around $4-$5. We usually drank water with our meals, or an occasional coke. We always had an afternoon snack of ice-cream or popcorn or something like that, and that sometimes cost around $3 a person.
You can check out some food prices here:
www.dlresortinfo.com/DisneylandMenus/dlmenus.html
It really just depends how fancy you want to eat, where you eat, and how much you eat!!
I found the prices on Harbor to be just as expensive at inside the park, so don't count on leaving the park for cheaper meals!!
We ate at IHOP one morning, and it cost us over $30 for breakfast! And that is with 2 kids meals and I just had toast and juice!!
Have fun!