I'll shamlessly plug a hotel I've been checking out for myself for some time, which is Hôtel Home Moderne, a 2-star hotel south of Montparnasse. It's "comfort" room for double occupancy is EUR75 per night and is available checking 26th Dec.

It's a bit of a walk to Convention Metro stop but I've read lots of reviews and the neighborhood rates as good and safe if unspectacular.

Just a thought since you were soliciting some ideas.