Shopping and food is really more expensive compared to Germany. There are many advantages for students, just ask every time if there aren't any.
At the ENS, I lived in the residence which might be the cheapest and definitely the easiest opportunity, I made a half, half (a bad and a very good) experience concerning the flat mates. Facilities there are medium.
high cost for accomodation (400-500) --> go to bejamin delessert ( good and cheap student├»s residence)
don├»t move in with older french woman who rent their flat--> they mostly just want to get money out of the situation
look for a good mobile card (orange, bougyues)
sign up for the programme jeune which is offered by your bank --> you get discounts