Just as a point of reference: the top 15 grossing apps in Japan at the moment are free to play but with in app purchase battle/card social games. I also work making them.

1-5 http://i.imgur.com/cuzGb.png 6-10 http://i.imgur.com/ckpIC.png 11-15 http://i.imgur.com/jzFWa.png

#16 is Line.