I stayed at the Hotel Cujas Panthéon in mid-September, and would recommend it for those looking for budget accommodations in Paris. Prices vary greatly throughout the year, as is probably true of most Paris hotels, so if we had gone during an off-peak time, we could have gotten a heck of a deal. Even so, we paid only 147 euros per night for a twin room and breakfast for two, I think not bad for that location: right in the heart of the Latin Quarter, a few blocks from the Panthéon, and only about a 10-minute walk to the Luxembourg Gardens. There is a bank with an ATM at the end of the street and a post office nearby. The room was small but clean, they have luggage storage, and there is air-conditioning and an elevator. The 24-hr English-speaking reception staff were very nice—helpful with luggage storage, obtaining a taxi, and restaurant recommendations. I would advise light sleepers to request a room that does not overlook the street—the bar at the end of the block can get very loud, and especially on the weekend, those students party all night.
I stayed there last May, and I would recommend it too. I booked it last minute when it was clearly not booked, and I paid only 80 euros/night. I did have a room facing the back so very quiet. I loved being so close to Luxembourg Gardens, perhaps my favorite spot in Paris. It is close to a RER B stop so a straight shot to/from CDG airport as well.