Create an event where you show some REAL appreciation for customers that spend, let’s say.. I dont know.. over $10K on the game, for example. Maybe 15K Crystals or even better, how about enough Zeta material to complete a few skills? I’m sure this little customer with my little wallet is insignificant, but I’m about at the point in the game (like with Vikings or Crusaders) where I realize it’s time to move on and waste my money elsewhere.
High spending customers would stick around longer and waste more money if you gave them some feel-goods occasionally.