I collected a little over 100 coins, but...

I've given up. You get coins whether you reach the goal or fail, but they keep decreasing (it started at 5 coins, then went down to 4, then 3). Also, it didn't used to be like this, but now there's an ad after every play, which is annoying. You can watch an ad to get 3x rewards, but that's probably like watching 3 ads, so the efficiency is questionable. I don't recommend this if you're aiming for action quests.