My answer is:
Not many.
Or, maybe some.
Quite a lot, if you're lucky.
Actually, I have no idea.
Some books sell and some don't. You will only find out if you try. Few authors actually make money from the sales of their books. Bear that in mind at all times.
What I'm saying, or rather beating you over the head with, is my advice not to base your publishing decisions on some sort of business model. You know the sort of thing. If I sell 200 books per month at £2.99 and get 70% from Amazon that will give me £5000 a year. So I need to write 10 books then I need never worry about money ever again. Yay!
I've tried this and, believe me, it doesn't work.
Books don't sell themselves. Promotion costs time and money and doesn't guarantee sales. Think of your own buying habits. Do you buy a book written by someone you've never heard of just because it pops up in your Twitter feed? No, neither does anyone else.
Publishing is a lottery, that's a fact. Unless you're an established author or a celebrity, there's no guarantee of sales. That's why conventional publishers are so picky these days. If they can't deliver a good return on their investment they're soon going to be out of a job.
If you self-publish there are lots of things you can do to promote your book but there's no guarantee of success. And it has very little to do with the merits of your offering. It has more to do with the quality of your cover and the first few words.
My advice is to accept that whatever you spend on publishing your book, and spend you will have to, bear in mind these priorities. Cover. Opening paragraph. Blurb (book description).
Promotion can get your book noticed but once you've attracted someone to your website or Amazon listing what they see is going to have to impress them mightily for them to part with their hard earned cash. What's not going to get their custom is seeing something that in any way looks inferior to the professionally produced books they are used to buying. Your book has to look at least as good as they do and maybe even better if you're going to get a sale.
And this means you are going to have to spend some money.
Which brings me back to the question. Even if you spend money on editing, cover design, production and distribution there's no guarantee you'll ever get any of it back.
What you will have is a product that stands a chance, that's all.
