Whenever I visit Melbourne I always go to Crossways, the Hare Krishna vegetarian restaurant at 123 Swanson Street, for a cheap meal. The meals are wholesome, tasty and pure vegetarian...no meat, fish or eggs...but even if you're not vegetarian, it tastes so yummy and the price is so good, that you won't complain. If that wasn't enough, it's also 'as much as you can eat', so if you've got a very big appetite this may be place for you. Last time I was there it was $7.50 if you're waged, and an incredible $5.50 if you have a concession card. Pretty hard to beat!
If you aim for cheap, I would suggest getting free promotions from restaurants. For something more tasty, you can try Asian restaurants or some fastfood shop. The Bread Top shops are cheap and quite good according to some people, with price ranging from 50c to 2$/pc.
DonDon on Swanston Street provides cheap and filling lunches from $7. Salsas Mexican restaurant is another great destination for cheap and tasty lunches for $5-$10. Sushi from any of the sushi outlets can ensure a healthy and filling lunch from $5.