You pay your own travel costs to get to the meeting point / camp and the placement fee. Food, accommodation and a small recreational program are provided in the project, but you should bring some pocket money in any case.
In some countries, especially so-called developing countries, there is often an extra fee, as most of these organizations are not funded by the state. As a rule, work camps for teenagers often also have an "extra fee" because of the higher level of organization and support needed. The amount of extra fees is included in the project description.
Your health insurance or visa may cause additional costs in some cases.