I am presuming you are looking for recommendations for specific restaurants where you can eat well on that "sewing thread" budget. We have lived in Paris and go there several times a year in our work. Here are my specific recommendations:
(1) Creperie Bretonne at 67 Rue du Charonne Don't miss the meal sized ham, cheese and egg savory crepe and especially the Salted Butter Caramel sweet crepe.
(2) Frenchie's To Go- at 9 Rue du Nil You can get it to go or eat it here. They have sandwiches like a great pulled pork and pastrami on rye, hot dogs, breakfast items, sticky buns, coffee and chocolate drinks and a not to miss non-alcoholic ginger beer. I like their lemon tea.
(3) Le Petit Cler- at 29 Rue Cler I like the beef stew, roast chicken, and the creme brulee.
(4) Chez H'Anna at 54 Rue des Rosiers. Falafel heaven! Plus curried chicken, kabobs, etc.
(5) Dong Huong at 14 Rue des Rosiers If you aren't a Vietnamese food fan already, you will be once you eat here. Chicken kabobs, Vietnamese ravioli, and my favorite- Bo bun cha gio which is a mix of noodles, beef, onion, and peanuts. Try the mandarin orange or mango sorbet.
(6) L'Eclair at 92 Rue Cler Go for the burgers, salads, omelets, quiche, brownies, and cheesecake.
(7) Cafe Pere & Fils at 86 Rue Montmartre The servings are huge and the prices tiny.
(8) Nick's Pizza at 24 Rue du Faubourg Montmartre Huge yet cheap slices of the best pizza in Paris
(9) Croq 2 Dent at 156 Rue Montmartre has cheap prices and generous sizes of soups, sandwiches and salads
(10) Breizh Cafe at 109 Rue Vieille du Temple has incredible savory and sweet meal crepes. For dessert, try a pear crepe or chocolate mousse crepe.
(11) La Creperie de Josselyn at 67 Rue du Montparnass is another savory and sweet crepe place.
(12) Le Cafe du Marche at 38 Rue Cler serves everything from burgers to beef stew to steak and pasta. Salads are good too. Desserts are wonderful.
You can also hit the grocery stores, bakeries, shops (meat, cheese, etc), veggie and fruit stands, and other places to put together your own sandwiches, salads, veggie, and fruit meals.