If you are staying a week or more, I would suggest the Zen Centre where we stayed in May-June this year. A couple of other ziners have stayed there. It is conveniently located, just 5 minutes walk or so from Bastille. We were able to walk to the Left Bank, to Notre Dame, and more. It costs 280 Euros a week, single. You can get extra days. There is a kitchen corner with adequate cooking equipment, private bathroom, toilet. Friendly quiet house. You must take off your shoes at the front door. No hotel services such as housekeeping. No elevator. http://www.maisonzen.com

It is listed in several guidebooks now, alas! I recommend it.

Frances Toronto, Canada