I haven't looked for it in a long time, but for many years, a popular cheap place for students and locals was something like Cafe des Beaux Arts (traditional French) on the corner of rue des Beaux Arts. There are a couple "formula" steak chains very close to St Germain des Pres that are reliable. Another one that I like, if it still exists, is Rotisserie d'en Face (simple grilled meats and other things, in French style), a medium-priced outpost of Restaurant Jacques Cagna, a Michelin-starred place in an old house around the corner.