I love the mechanic of players mining resources and the game being the middleman. The dynamic pricing should be really interesting.
I hate the withdraw fee. I don't see any reason to burn 2%. And I don't think having a super high withdraw fee for newer players is going to encourage growth as much as it will encourage not playing. Make the withdraw. Make the withdraw fee 2%. half goes to castle owners.
Don't make the Splinterlands mistake of punishing lower level players to encourage them to move up. And don't make their other mistake of skewing rewards so heavily to the top. Most of your player base will never be a top player and they know that.
Top players on the other hand will push to be there because they want to be the best more than they will for direct financial incentives. That said, everyone paying the king is another bad idea.