A good, affordable option is the Schanzenstern hotel, offering rooms from 39€ per night (excl. breakfast). It is located very close to the Sternschanze train station (map), from where it is just a two-minute train ride to Dammtor. If you like, you can also walk directly to the conference venue, which should take around 20 minutes. Alternatively, you can use a city-bike that can be borrowed for 30 minutes without charge.
Other good hotels closer to university but more expensive:
All are within 10 minutes walking distance from the university Hauptgebäude.
If you wish to book a room with any of these hotels, inform them that you are a guest of the Philosophy Department; this will likely result in a slightly discounted rate.
We would advise you to book your accommodation well in advance, since the Hamburg Triathlon is taking place at the same time as the conference and may attract many visitors.