for poor uni students - i like to go around city food courts at around 3pm, 4pm.
MLC Centre clears everything out pretty much at 4pm and you can pick up a good meal for 5 bucks.
But as for generally - and I know this is uncool - but after 5 years of uni i've realised the healthiest, most cost-effective meal is the home prepared one.
I assume you're a young'un. Invest a month to learn to make good food or learn to make food you'll enjoy that'll also travel.
You will save thousands over a 5 year course. A conservative estimate would have the average uni student eating out 3 times a week for lunch spending a few grand.