It's not meant to be something you can buy (or buy out) quickly - we're in a marathon (to learn Japanese), not a sprint. We hope that the garden will provide a goal that you can work on for months and years, not something that can be quickly cleaned out :)
Keep studying and you'll see that you'll have even more kao coins than you can think of. I'm currently at 1,341,450 from simply studying and doing all my daily/weekly quests, keep doing them and you'll see how fast you'll earn.
If you go down below the quests in your garden, you can filter out quests you don't enjoy. Then work on maxing out your daily quests and chipping away at your weekly quests.
Keep studying and you'll see that you'll have even more kao coins than you can think of. I'm currently at 1,341,450 from simply studying and doing all my daily/weekly quests, keep doing them and you'll see how fast you'll earn.
