It really depends what you want. There are two food courts on campus - Wentworth and Manning, and they both have a variety of food (kebabs, thai, japanese, mexican, indian, sushi, cafe stuff, pizza, etc). Everything has different prices but you're probably looking $5-10 for most things.
There are also a whole lot of coffee carts that sell things like pies/sausage rolls/coffee and the like but I wouldn't exactly say they're filling.
A lot of people go off campus to places like Newtown for cheap thai ($6.50 for a very generous portion).
You could also consider making your own food and bringing it to uni.