Accomodation

Charming hotels
Michelet Odéon Abbatial Saint-Germain
Les jardins du Luxembourg Dacia Luxembourg
Luxembourg Sorbonne
Grands Hommes Notre-Dame
Select Best Western Jardin de Cluny
Grandes écoles

Nice hotels (from 65 to 90 euros a night)
Senlis Collège de France
Trois collèges Le home latin
Chaplain rive gauche Cluny Sorbonne
Monge Saint Jacques
Mines (they also have triple rooms)  de France
Familia (they also have triple rooms)

Less expensive hotels
Port-Royal (some rooms at 41 euros a night) Beauvoir

Hotels in Montparnasse
(need to walk about 20 minutes, via the Luxembourg garden, or to take bus 89)
Paris-Odessa (they also have triple rooms) Central

More hotels with these links
The selection from the House of mathematics TimeOut
France.com Venere.com