I've upgraded the crystal mines many times and just realized they are currently full in 36 mins. What's the point?! Why bother if they offer so little reward as your characters reach higher levels. Unless you are constantly checking the game you can only upgrade a character a few times after being away all day. I don't remember previous versions of the Maze having a limit. You could come back hours later and have tons of crystals to spend. Without spending in-game currency or real money you are going to hit a ceiling quickly. Once you can't win any more battles the event turns into just slowly farming for crystals.
I also agree about the boosters being too expensive. They used to cost 1 star during the first Maze event. Then they cost 15 stars in the second Maze event. Now they cost 300 stars which is crazy!