Someone who has moved on to another board has stayed at Hotel Delavigne in the 6th, and recommended it several times. It's about 130€ for a twin and is in the St Germain area (6th).
http://www.delavigne-paris-hotel.com/

Also, a good, central hotel is the Hotel du College de France in the Latin Quarter.
http://www.hotel-collegedefrance.com/

Another one near Montparnasse, and metro Vavin, is the Hotel Delambre, recently renovated (a couple years ago).
http://www.delavigne-paris-hotel.com/

Hi Allie allie:
My husband and I have stayed in Best Western Hotels all over France. BW acts as a booking agent for the hotels. They are all usually full of local character and in good locations.

We haven't stayed at the Acadia Opera but really love the Folkestone. It is so close to the Opera, the Metro, the Madeleine, Le Printemps and Galleries Lafayette. We walked everywhere from this location including the Louvre and rue de Rivoli. If your Mum has difficulty getting around it is in such a good location. I loved the restaurant across the street (I am trying to remember the name of it.. something sunny I remember.. St Tropez maybe)
Anyway don't worry about staying in the BW's. They offer good value and local flavour if not "bougainvillea covered walls" and "quaintness".

I use the already mentioned www.venere.com to check on hotel availability. VERY user friendly site with the ability to just look at hotels within a certain area or to look for hotels available only during your stay. There are also many reviews of the various hotels which help send up red flags.