We got a good deal for Ibis Tolbiac on booking.com, for 45 euros a night. It was clean & basic, and since we just needed a place to crash at night, it turned out to be good value for money. It's location near the metro station is ideal, and the bakeries / cafes in the vicinity are really good since they are far from the more touristy areas near the main attractions.
Overall, I'd recommended it for a budget stay.
My travelogue from Paris: http://theshootingstar.wordpress.com/2011/05/11/paris
