on Just Publishing Advice:

When you sell your ebooks with KDP, Amazon takes at least 30% of your selling price. In some cases, it can take 65%. On top of that, you might even lose more due to downloading fees.

If you sell your ebooks with Smashwords or Draft2Digital, both take around 10% of net sales. It means that if you sell an ebook on Apple or Barnes Noble, each takes 30% of a sale, and then your aggregator takes 10% more.

Then there is the problem of withholding tax if you don’t or can’t register with the US tax office. In this case, you lose 30% more from each sale.

You can’t avoid all of these charges and difficulties when you self-publish. But there is a way to do a little better.