My favorite thing to eat on a budget at DL is the Chicken Fusilli at Redd Rockett's Pizza Port in Tomorrowland - it's yummy and filling!! Top it of with a giant star-or-moon-shaped rice krispie treat and you'll be in heaven.
The skewers at the Bengal BBQ in Adventureland are a great snack...they're not super filling for a meal, but they're a great way to keep full between meals!
Also, DH and I love to hop over to Downtown Disney and get tacos at the quick-service window at Tortilla Jo's. It's quite a bit cheaper than anything you'll find in the parks, and it's fast and delicious...and you can hop right on the monorail to get back into the park!