You will find yourself using MANY MANY coins over the course of the game. Evolving 5* girls is 150k coins the first time from memory and 6* are 250k then it's extra for each Manyu you feed them and the higher the girls level the more it costs. I have found from experience that 10,000,000 coins does not last long when you start a serious girl / equipment upgrade cycle and more often than not i'm below 1 mill coins at all times these days.