Of course, there is no magic formula for what to spend!  However, there obviously isn't much you can buy.  It would be smart to buy raw ingredients, because there is a lot of added value due to the cost of service in restaurant meals.  Going to a local Chinese fresh market is the best option to ensure quality of food as well as a cheap price.  Try and communicate with the farmers, and maybe even bargain for a lower price!  It's not easy, but it's possible.  Try and have a balanced diet - the correct proportion of carbohydrates, proteins, lipids, vitamins, minerals, fibre, and water.  Don't go to supermarkets or restaurants!