There are a couple of hotels which are not too expensive and rather close to the conference venue: examples are the Super 8 Freiburg (15 min distance by public transport) and the Motel One Freiburg (5 min by public transport).
The cheapest Hostel we found is the Black Forest Hostel. Using public transportation, it’s only 25 min from the conference venue. Its prices vary between 44€ (single room) to 26€ (four beds dormitory) or even only 23€ (6 beds dormitory) per night.
Other options are available here: https://visit.freiburg.de/en