Next to nothing: 100 euros (including 2 weeks of accomodation).
Most of this money is used to pay for your accommodation. Which means that your room costs you just above 7 euros/night. A small fraction of this money is incorporated into the general budget of the school, you can think of this fraction as a small compulsory donation.
The general budget is used to pay grants for travel and accommodation to students who could not afford to attend otherwise. The budget also pays the teachers' accommodation and we subsidize their travel. We have no other costs thanks to the fact that the school is entirely self-organized by volunteering linguists.
(If paying 100 euros is a problem for you, there are grants to cover this cost.)
Almost nothing: On-site life is cheap
Nothing: There are grants