Since everyyone gave suggestions for outside the park, I will share my tips for inside. We are at one of the parks every weekend so we have to eat cheap, lol. All of the TS restaurants are really expensive and we just can't afford them.
Regarding Seaworld and Universal....I don't really like any of the food at those. The Seaworld all you can eat deal is a good value but the food is bleh imo. I would eat snacks and then go outside the parks to eat. I have heard Mythos at IOA is wonderful and not too pricey but I've not tried it yet.
Disney has some really great deals if you know where to look. I will often buy a kid's meal for myself at QS locations. Skip the chicken nuggets, they don't give you many but the mac n cheese and pizzas are good. I will also share an adult meal with my child a lot.
Some QS places are better than others but you are still looking at around $10 a person. But keep in mind you can bu a la carte even though the menu onl lists a whole meal. You can get a burger without the fries. And order ice water (always free) instead of soda.
At MK, I really love Columbia Harbor House. They have a grilled salmon with cous cous and broccoli for like $10 which is really nice. Their mac n cheese kid's meal comes with yogurt and grapes and plenty enough for an adult. Plus the place is very quiet, especially up stairs and it's a good place to unwind.
I've also heard Pecos Bills is nice and Tortuga Tavern. Both are supposed to have nice toppings bars.
At Epcot, Sunshine Seasons is a really nice QS location and you can refill the sodas there.
But even with that, we rarely eat QS. We mostly snack all day. Disney has some very delicous snacks. The bakeries are your friend! A lot of them have really delicous sandwiches for around $6. Get that with an ice water. Also, the muffins and cupcakes are HUGE. Check out the Disney food blog and search for snacks and where to get them. There are definately some must do's.
Pics of a cupcake and one of those $6 sandwiches
here
You can also bring snacks into the park.