I'm going to give some tips, for that coin, an idea of mine.

1- Each game should have a currency difficulty, the most difficult type games the pyramid reward easier than the Flying.

2 - In the game flying should put 2 things, a solution would be every 100 Score a coin ai did not give a lot of money, or else put a competition who makes the highest Score within 5 or 10 minutes wins an amount.

3 - You win a lot of money and great, but there are people who do not accept wins a lot or bull up the game or the currency, earning% on top of that, So I advise you to put lower reward then you will have no problem.