The Latin Quarter is packed with great, cheap restaurants. (Almost every other building seems to be a restaurant or bar). As well as French cuisine you'll find Indian, Chinese, Thai, Greek, Italian and many other cuisines (and plenty of burger bars and fast food outlets as well). A good three course meal costs from as little as �10. (It's generally better to fill yourself up with three courses than to snack here and there. A single course often isn't much cheaper than three courses from the various set menus on offer).
To find all of those great eating places take an RER train (on Line B or C) to St-Michel Notre-Dame. (Alternatively take Metro Line 4 to St-Michel, or possibly Line 10 to Cluny La Sorbonne). Simply wandering around will probably bring you to the restaurants but, if you want a focal point, head for St S�verin church (which should be marked on any decent map of Paris).
Chris