At least once a week I go to Caldo Freddo at 34 rue Montergueil.  This great italian place is not only cheap and well located, it also has what could be the best pizza slize in Paris. The guys working here are supernice and the owner is, judging from his accent, probably italian. If you’re not up for pizza they also have some great paninis and salads. It’s the best place for a quick lunch if your close from to halles.