Some farms and NGOs charge an extra fee from volunteers in order to cover costs with accommodations and meals. This information must be written on their profile – in case it isn’t there, let us now on email@example.com.
There are also some hosts in the hospitality business that charge a deposit to guarantee that volunteers will really show up. Normally, these are hosts that have already suffered with no-shows, so they ask for a deposit as a protection measure. The amount is generally given back at the end of the experience.