And royalties are even less for discounted books such as those sold in supermarkets.
You also have to take into account agency fees. In the UK for fiction books, those are around 15% plus some expenses at cost for stationery, phone calls and the like. If your book is sold abroad, it's about 20% split 50-50 between your agent and the foreign agent. Anybody who thinks writing a best seller will take the author from rags to riches needs to have a good think. The vast majority of authors earn next to nothing and that nothing has been dropping quite dramatically in recent years. It's a tough game. To retire early, an author needs to write a million copies sold best seller, and the odds of that, while better than winning the lottery, are still pretty slim. Each publisher or imprint typically only publishes a handful of books each year, and only a tiny majority of those will sell well enough to make an agent love their author.
I suspect the real objective of any author/agent combination is to sell the book's rights to a production company. If they achieve the almost impossible and find a TV or film deal, then you really are in clover.
The royalties are low for many reasons:
First of all, a book shop may receive a 30 to 35% discount on the sale price of paperback fiction. At least that's how it was when I worked for a large bookshop chain back in the late eighties. Larger chains will command 50% or more, sometimes 60%. I think supermarkets are around 75%.
Ebooks are different again. From what I've read, authors keep 50% to 70% of the download price for those, but since many are free, then that's not a lot for most authors. And that is pathetic, frankly.
Then there's all the people involved in marketing the book. Designers, editors, production planning, every book is a serious project that costs a lot of money. And you'll be expected to help out, with a web presence, author signings, and so on.
I think allowing 50p per copy sold will give you a reasonable minimum estimate of what you can expect after everything is deducted, including taxes. That's not as bad as it sounds. If you're lucky enough to sell a million then that's 1/2 a million in the bank. And a long lived book will rake in royalties for years. George Orwell's 1984 is a good example of that. It sells around 1,000 copies every month. So, £500 a month, £6,000 pounds a year. Not bad for something written back in the forties.
Think of it as selling widgets. You pay your time and maybe money upfront for one copy , then you get a slice of every single copy sold after that.
That's for the UK. America is different. Perhaps somebody else can chip in on that one.