There are loads of great cheap eat places across Manchester. In the northern quarter, I like Hunters - which does adventurous and lovely curries. In fact it was recently voted top cheap eat in Manchester by the Guardian http://tinyurl.com/5fzv4e.
I also like The Northern pub on Tibb street - especially their Sunday roast which is delicious and always delivers portion wise. Nearby you have Cup - a cute little cafe owned by DJ Mr Scruff. They do homemade cakes and organic stews. Yum.
Little bit further out of town in Withington (about 20 mins drive from the city centre) is a great place called Aladins. Great middle east cooking and it's a BYO (bring your own) so makes it even more wallet friendly.