Second to your air fare and accommodation, the single most expensive thing you will do in Paris is eat out, A city with some of the most heavenly options when you are an epicure, the upscale restaurants in Paris are some of the most expensive, and before you know it you have spend a lions share of your travel budget. If you are not on a budget and do not need to watch your spending, seriously indulge in the best Paris restaurants like Arpege, Grand Véfour, Mélac and other culinary chapels, but if you are a backpacking student or strapped for cash, you need more affordable options. You need to watch prices, get good portions for your money, and take out some times. The high profile options do not feel as good on the pocket as much as they do on the palate.
In any case, eating on the cheap when you are in Paris far from implies that you eat unhygienic or badly cooked food at squalid establishments; there are quite some options you can choose from to dine well, and cheap. Plenty of locations where you can get good food for less are in Paris, you just need to be in the know what to look for, and where to look for this.
The best locations to head for if you want to eat affordable as well as decent fare, is any place known for being a tourist haunt. The more the number of tourists that teem in a location, so much higher will be the food prices. This is because the vendors charge what they like to tourists, as they know that tourists are generally uniformed, and reluctant to go searching for other options in places other than the main tourist spots. You need to take the paths the locals take, and not the tourist filled locations where traders are on fleecing sprees. Explore a bit, ask around, make friends, the only way to avoid having to pay the tourist prices is to be in the know. You can ask students you may meet as well, as they are generally in the know of cheap and fun places to eat.
The Marais area is quite a tourist populated area, but is also where the Jewish quarters are, so the area is populated by a large number of falafel stands, which make for excellent options when you are starving. Chick pea falafel stiffed in Pita bread, falafels are served with all the fixings. Delicious, crispy and filling, falafels make for a heavenly snack. There are numerous stalls that serve falafels in this locality, and you can compare prices to find the cheapest ones. Kebabs are also served in the stands, which is another tasty option.
The Latin Quarter is another tourist location, however the large student population makes for some affordable options especially the crepe stands. Crepes taste excellent, and are famous as a late night snack with pub crawlers. There are some excellent night clubs in this area, which explains the multitude of crepe stalls.
The cheapest options when it comes to food in Paris do not include French food but Asian, African, Jewish and other ethnic food. This is typical of many big cities which are melting pots of cultures. Couscous is a delicious and satisfying option both in the day as well as night, and Morrocan and Lebanese couscousseries are the places to head to.
Chinese and other Asian restaurants are located in large numbers in many places in Paris, but the Asian fast food delicatessens are where you should go to get some extremely inexpensive food. Food is sold here by the gram, and these take out joints make for some swift, cheap and substantial portions. Hardly gourmet fare, but in no way bad.
Indian restaurants, especially south Indian restaurants are also good bets, where you can enjoy paranthas, pancakes with stuffing in them, delicious and hot.
The falafel stands are also other examples of cheap ethnic food that can e enjoyed in Paris. Crepe stands are the cheapest options for French food. These are excellent for budget meals.
Another famous Parisian item is the croque monsieur, which consists of a ham and cheese sandwich, which is grilled and has cheese outside the sandwich. Croquet madame is a variation of this, and the only difference is the egg on top, for some protein fix. Delicious and messy, these are cheap as well.
In winters, roasted chestnuts make for nice warm snacks, and it feels good to much on these.