The network takes a 20% service fee. This is the only fee charged by Mysterium, which is used to continue building, maintaining and improving the service. Its purpose is similar to other peer-to-peer platforms, like Uber or AirBnB. However, in this case it’s our smart-contract automating the payouts and fees.
Without a percentage cut, the node network must be maintained by a team voluntarily (for free), which means quality would go down and general network growth very slow.
Note that you will also pay blockchain transaction fees when you withdraw your earnings using a Polygon Network. However, the fees are usually minimal and typically it's only a few cents at a time.