I self-published a romance book under a pen name last September. All in, I spent around $600 to publish the ebook and print version. To date, I've made around $10k. Like another poster mentioned, romance is a great genre for kindle. I've spent $0 on marketing, but people keep finding my book. Have you lurked in the KBoard forums yet?
http://www.kboards.com/index.php?board=60.0I can't speak much to non-fiction publishing, but some bits of advice that've worked for me:
1) Make sure your cover and blurb are really great. They're the best advertising for your book. I picked up a fantastic cover at goonwrite.com for $45. He'll even create the front and back covers for createspace for an extra fee. I also recently heard about this new program called canva where you can make your own cover for free.
https://www.canva.com/create/book-covers/ Whatever you decide, make sure your cover and blurb are as kick-ass as can be.
2) Write all of your books first. My big mistake was writing and publishing one book at a time. I'm not a fast writer, and I can't pump them out like many romance authors can. Now I have readers waiting over a year for the next book, which kills momentum. The best marketing you can do is to publish another book. Frontlist sells backlist. If someone likes your new book, they'll want to read your older books. Make sure you have a backlist they can buy. If I could do it over, I would have 3 books completed, then publish them 3-4 months apart.
3) Start a mailing list. Free on mailchimp. Put a link in the backmatter of your book. Build up your list.
4) Don't be afraid to give away your book for free, and I don't mean making it free on Amazon. I've heard very mixed results from making a book free on Amazon. You'll get lots of downloads, but will anyone actually read? Instead, I searched out bloggers and reviewers and readers in my genre. I offered them a free copy in exchange for a review. That started the word-of-mouth that still continues for my book.
This is just what I've learned from my experience and is probably different for other authors. Advice is not one-size-fits-all, so YMMV. But overall, I've loved self-publishing and I highly recommend. It's helped me tap into this entrepreneurial side of myself I didn't know existed.