Ok, this is probably one of the best cheap food deals I've ever come across in Sydney... it's $1 hotdogs on Tuesday evenings at The Soda Factory. Yum yum!
One of the reasons why I like this place (besides the cheap dogs) is that they open late. I find that it can be hard to get cheap food in the city after a certain time (say 9pm). Some restaurants close quite early too. You can always grab a bite at a bar but sometimes the noise in those places can be quite overwhelming so I think this venue is a good compromise.
Location-wise it's pretty swell too. It's just a five-minute walk from Hyde Park in the city on the Liverpool/Oxford streets side. Plus you can always head up to Oxford Street to check out the nightlife after that!