This is hard imo. Sure you can get EvilFood(tm) from some of the fast food places around the place. But I don't want to get fat and die young.
DonDon is one of my fav places (http://www.urbanspoon.com/r/71/1488361/restaurant/CBD/Don-Don-Melbourne)
I know this question was mainly talking about pre-made food.. but ... I'm still finding that Coles supermarket (either near Southern Cross Station or the one near Flinders on Elisabeth Street) have food you can make up. Sometimes they have (hot) roast chicken on special.
I like Don Don on Little Lonsdale for bento boxes ($8.70), Jimmy Grants in the Emporium food court for lamb or chicken souva ($10) and Mamak on Lonsdale for roti and satay sticks ($6-$10).
We head out to B3, they specialises in great-value baguettes, making it the perfect place to grab a quick bite on the run.
Pick up a ham and cheese or vegetarian baguette as you make your way from shop to shop or the office. Or try the smoked salmon, brie, flake, chicken or eggplant schnitzel baguettes
Kalamaki Greek Street food is a fantastic option. We found this place one day just wondering, looking for a snack and came across it. It was packed out! it has great kebabs which are traditional greek style and less than $10 and also comes with traditional cola and chips with fetta. A great visit.
I am a big fan of this teeny tiny character filled pattisserie at Le Caffe Le Petite at Gordon Place. Every day the French pastries are baked fresh every day. Grab a salad baguette or a home made sausage roll with an exceptionally made coffee sit down - all under $10. The most quaint delicious place in Melbourne