Verified answer

Collecting stars is the event of the game series. Normally stars costs 20 coins but in some boards them costs 10 coins. I don't know do they cost more than 20 coins. In DS version stars can cost 5 coins but only in event of final 5 frenzy. Winning minigames gives you 10 coins (it depends of a minigame. In some minigames you get more or less.) But yeah you should buy a star. And after a board if chosen bonus stars before starting a board you can try to earn 3 stars. But enemies tries to earn bonus stars too. Bonus Star goes to a player who did the most events of that bonus star. Example: hit most to happening spaces to earn a happening bonus star. Bonus stars will be chosen random so try to do many things as you can.

Hopefully helped. :)