Specific to Paris (and vegetarian food) : There are many Indian restaurants next to Gare du Nord, on Rue du Faubourg Saint-Denis, which has a significant Tamil presence. Passage Brady has many Indian restaurants too. Quality can vary, but I specifically recommend two restaurants, Sangeetha and Saravana Bhavan, both Chennai-based south Indian vegetarian restaurant chains. For the best value for money, try the mini tiffin at Sangeetha - a (smaller than usual) masala dosa, idly, sambar, chutneys, pongal, rava kesari, and filter coffee at only €7 (yes, that is a good price!). There are other cheaper restaurants in the vicinity, but they aren't that good.
(Mini tiffin (coffee not shown))
Even cheaper, there is a place called Apna Punjab, which serves vegetarian Punjabi food. It's just next to the northern terminus of Metro 7, rather far from central Paris. The decor and furniture is basic, and the restaurant looks like it could very well be in a small town in north India. A full meal consisting of 3 rotis or 2 nans, with a sabji and dal (and perhaps rice? I'm not sure), is only €5, while chhole bhature is €3.50. If you're in Paris for only a few days, it's probably not worth spending the time to go here.
There are several north Indian restaurants in Paris, at Passage Brady and other places, I don't have much to say about them.
Even if one doesn't want to experiment much with food, it's worthwhile to try some other foods which are very close to Indian in taste. For example, Lebanese food can be quite similar to Indian food: try the falafel (a kind of vada), at Maoz Vegetarian (sandwich with 5 balls of falafel, and salad from a buffet at €4.90), or the rolls ("manakiche") at Chez Le Libanais, particularly the "batata" (potato) which could pass off as Indian (€4.20).
Then you have Mexican/Tex-Mex burritos - a large chapati, rolled up, and inside it rice, rajma, possibly meat (not necessarily), some vegetables, salad, cheese, cream, and sauce (can be spicy!). Typically this is made in front of you, and you can ask them to add or not add specific items. I'd recommend BocaMexa and Chido on Rue Mouffetard (€6.50 - 7), or Chipotle (better than the previous two, and at €9.30).
You could also try Subway - a sandwich with your chosen subset of tomato, onion, peppers, olives, cucumber, cheese, lettuce, sauces, etc, and various kinds of meat (30cm vegetarian is €6).
If I had to pick just one place, I'd pick Sangeetha.
All these places are open all days of the week, at lunch as well as dinner times. The prices are as of April/May 2015.
Avoid buying beverages or any packaged food such as chips at restaurants, they are expensive - buy them cheaper at a supermarket instead.
Enjoy your trip!