Are you in a higher league and trying to enter a tournament in a much lower league? Many lower league tournaments have cheaper entry fees for players in the appropriate leagues and higher entry fees for higher league players. This provides an environment for newer players to have the opportunity to build up their decks instead of being crushed by experienced players from higher leagues.
Please note that for each game format (Wild or Modern) the entry fees are based on your leaderboard league at the time of sign-up. If you are in the Diamond leaderboard and drop to the Gold league because your rating drops, you are still considered Diamond for entering tournaments. However, if you advance leagues after you enter a tournament, you can bypass the higher entry fee because the system only checks your leaderboard league when you sign up, not when the tournament starts.
Also see these related tournament-related articles:
Participating in Tournaments (Events)
Why was my tournament prize split?
How To Create a Splinterlands Tournament?
Why Didn't I Receive My Prize in Tournaments?
Why Don’t I Have Any Battles for the Tournament That I Entered?