Okay I'll express it in pounds
If you can speak some basic mandarin, and can recognise the items on the menu, then you can go into any old place and have a very good three piece meal including a beer or two for i'd say about 40 yuan which is £4. if you want to go a bit more upmarket and eat in a place with higher hygiene standards, and better presentation, it won't be that much more - i went to some fucking cool ass place which looked stupid and it was only 75 yuan and from what i gather that's a decent price for a mid-range restaurant for one. i had a main course, a starter, rice and a beer.
in supermarkets though, well, everything can be up to ten times cheaper than the UK equivalent unless it actually COMES from the west, in which case it costs almost as much. for example, a box of any kind of western cereal would be from 40-100 yuan, where you can find it any, while any kind of box of chinese porridge, savoury or sweet, would be much less. eating out in general is much cheaper especially for the westerner who doesn't know what is up and is likely getting ripped off.