I aim for quantity over quality. I pay to have books ghostwritten for a particular market/niche. I also acquire rights to other self published books and re-release them. Self publishing is all about the marketing.
For ghostwriting I focus on a couple different niches. I work with a company to get the books written. I write for the audience rather than what interests me. If vampires are hot one month I put that out. If it's billionaires the next I do those. I have a couple different "authors" that I write under that stick to each genre (One only writes LGBT Romances for instance). A large part was building up the email lists of readers and managing some social network profiles. Once you build up your following it gets much easier. I know some authors that churn out a book every 2 weeks and people will spend the $2.99 (or read it on kindle unlimited) every time. My release schedule is a lot slower, but I'm trying to find more reliable ghostwriters to speed it up.
Regarding the buying of book rights. I have a publishing company I run that looks for books that are not 'successful' and I make an offer. I'll clean up the book and re-release it under a different author name and do a little marketing. These I aim to cover costs before end of the first year.
I use vellum to format ($250 one time). Requires a mac. The company I use will sometimes do this for me for free.
Marketing beyond email list is then either book bub if I can get in or amazon ads.
Mailchimp/digitalocean for email list/Wordpress site hosting. Google Apps for email. Those are my reoccurring costs.
Break even is usually 800-1k copies sold @ $2.99 list (I earn 70% of that). Plus whatever I spent on marketing. Anything outside 2.99-9.99 Amazon will only pay 30% royalties. I earn roughly a dollar per book read via kindle unlimited.
The first few books will take a while to earn back. Building your following/email list is the most important thing. I use it for engagement. “New book coming soon! Giving away 25 advance reader copies enter here”. Then before release “pre order now. Here is the feedback from those that won a free copy.” The surge of reviews/early sales helps boost Amazon ranking (new readers) as well as to quickly earn back what I spent producing it.