Thanks, Eric for your insight. I think you're probably better off writing for the German market - it may be smaller, but definitely less saturated.
My own experience so far:
Inspired by the OP, I began seriously writing and publishing on Kindle five and a half months ago and have now published 31 books and counting. I am a writer by profession so I have written all the books myself, simply because I cannot afford to pay to have books written for me. I have published both fiction and non-fiction, across several genres and niches. I have done absolutely NO marketing other than the KDP program, NO social media promotion and NO fake reviews/review exchanges etc, and my sales last month just touched the $400 p.m. mark. My aim is to increase my monthly sales by $50 each month by writing and publishing more books each month.
As the OP said in his original post, this is HARD, HARD work up front, but once you've done the work, you will reap the profits on autopilot for years to come.
For the past five years I've tried various IM methods - affiliate marketing, blogs, adsense etc, but so far this is the ONLY method that has really worked for me. I know I'm not chalking up the huge sales that others have reported, but for me this has been a great start and as long as I grow my sales each month I'll be happy. My goal now is $1000 per month, and after that? Who knows? The sky's the limit.
P.S. Where are you, OP? It would be great if you could look into this thread every now and then and tell us how you're doing and how many books you've published so far. You've been a great inspiration to me and many others here, it would be nice hearing from you again.
The figures of the OP are highly inflated. Kindle sales look more like the above. That's realistic! I have about the same books, all nonfiction, and I write them myself.