Berlin has many hotels and guest houses at reasonable fees.

Room reservations should be made early, since July is a busy month with lots of tourists.
The public transportation system in Berlin is excellent and at reasonable fare, one way regardless of distance is 2.70 EUR.
You can take a hotel also at some distance from the conference site. Just make sure it is close to a subway or S-Bahn station.

see link: Berlin public transportation Tickets  

Participants who want to be at a very comfortable place close to the conference site can book a guestroom at Seminaris Campushotel (see link)
A small number of rooms have been reserved for that purpose.
The Freie University of Berlin gets special conditions. When you make a reservation mention the keyword “PEB 2016” to obtain these conditions. The room fee will be 80.- EUR per night including a breakfast (which is actually excellent).

Another housing option with good offers is a Guesthouse & Hotel, Jugendgästehaus und Hotel St-Michaels-Heim (see link). It is well connected via public transportation to the conference venue. Bus line 110 goes directly to the closest subway stop Dahlem Dorf (route)