One thing to consider is royalty rate. If you sell 5000 copies for $20 and earn a dollar per copy, you've earned $5,000. (This math is based on 10% royalties on the publisher's portion, which is about half of retail. And no, fiction books don't sell for $20 typically, but I'm using this number to make the math easy.) Now, if you sell 5000 copies of an ebook for $20 and earn 80% royalties, you earn $16 per copy, and e…
Depending on audience and how you advertise to them, I've seen people be really successful on Patreon. I outlined some of that in a different comment here earlier, but $15k+ a month (from Patreon alone, not including other publishing that you can also do) is achievable and I've seen it more than once on somewhat esoteric genre fiction at Patreon.