Department store and shopping mall food courts offer the best value for the money in dining for visitors, and the sanitary standards are generally high.  Here, you can spend as little as US $2 for a filling meal of pan-fried noodles and soup.  It's a noisy and sometimes chaotic dining experience, to be sure, but one that is authentic "modern China" and great for travellers on a budget looking for a quick bite to eat. 

Megabite is one food court that is highly recommended for quality and variety.  They have locations in the Science and Technology Museum, Raffles City Mall, and several Carrefour stores.  You first purchase a debit card and then check out the various food stations.  All of the vendors have a display of their offerings along with prices, so all you have to do is point to what you want and hand the clerk your card. 

Even cheaper eats, and not less nutritious and tasty are the endless "street kitchens" and hole-in-the-wall food shops found tucked away in alleys.  One particular area near Yuyuan is Sanpailou Lu and Sipailou Lu (stone's throw away from Yuyuan Gardens).   There are stalls selling barbequed meats and squid, steamed/fried meat filled buns with a soup (not xiao long pau), fried/steamed dumplings, fried/soup noodles, delicious cold noodles, candied/fresh fruit ... all costing under $2.  

In the evenings this area also caters to Shanghai residents who may not have the time to cook dinner, so you can expect an even wider spread.   An entire sumptiously roasted/braised duck can be had for all of $7-8!   In fact, these stalls capture the real taste & essence of local food fare, not to be missed.