A parisian friend booked me to this hotel Eldorado in 17 th arrondisement and I warmly recommend it to anyone that wants to stay in interesting neighborhood close to everything and not to pay much.
Hotel has rooms starting fr. 25€ to 65€/ night. Everything is very clean though old. This place has great ambiance and very friendly staff. Bar and restaurant are full every evening and the food is great. There is a beautiful courtyard where you can eat in summer nights.
Forget the left bank, 17th is place to stay. Lots of great restaurants and bars close.