User blog comment:Thumbwrumbler82/The rip offs/@comment-11494103-20131112113833/@comment-21232-20131113001755

Good point - do NOT level up over around 10 or 11 until you have the Move [monster name] to Gold Island goal, after which the first monster of each type that you level up to 15 and place on Gold Island*  will earn you 1,000,000 - 3,000,000 coins. Most will cost you more than that to get there, but it does let you build up a band of all non-etherial monsters in a different tune. There's more to this game than coins or I wouldn't bother with it.

*Note - while they appear on Gold Island, the monster you fed up is still on the island you bred it on, and still earns coins and takes up castle space there. Gold Island is strictly a display arena. Arrange the monsters as you see fit and enjoy the song!