Usually it's like this: If you do research in cooperation with a commercial (i.e., no NGO) organization or company, or if you do contract research, then you (or the company) has to pay a usage fee, so we can re-finance the server and further development.
If there is no commercial organization involved (and you don't earn money with the survey), you can use www.soscisurvey.de for free. Depending on whether the results are published, the questionnaire may display advertising on the last page (no publishing) or no advertising (only published research is really academic research).