Is it permissible to subscribe to a website monthly for 20 riyals to play tournaments on it and win a prize of 500 riyals for the winner, knowing that the subscription is only to identify serious participants?

It is not permissible to participate in competitions that require payment of fees, because that is forbidden gambling. The claim that participation is to distinguish serious contestants does not change the fact that it is gambling, for the participant is either a winner or a loser. Similarly, it is not permissible to take prizes in electronic games because of the Prophet, peace and blessings be upon him, saying: "There is no prize except in a hoof, or an arrow, or a hoof," and these [games] are not among these three.