If you want somewhere really cheap and yet classy, there's a lovely resturant on free school lane - the lane running behind corpus christi - (can't remember it's name, but there's only one of them, so you can't miss it); it does proper main courses for £5/£6, and has a real charm about it.
Slightly more expensive, but not massively more, is Bella Italia on Newnam road, opposite the Granta pub (along from the crossroad near Queens college). It's a converted mill over the mill pond, and actually has part of the old mill on display. Very historical.
The Anchor, on Silver street, is probably the best pub, with reasonably priced meals, good views, central location and very fast service.
Hope that helps; if it's any comfort, if my parents were coming up, the very last place I'd want to take them would be formal hall, what with all the mint races, drunken freshers and 'air strikes' that take place the moment the fellows have left.