Today, I was asked by a writer friend, who'd been offered the choice of a percentage of royalties or a flat fee for his book what he should do.
The book that has sold the most copies, took 2 weeks to write, but was my own concept. It has sold in the tens of thousands and is still in print over a decade later and still selling.
I was paid one thousand pounds to write it and it's made the publishers many times that amount.
There are days when I regret not pushing more for royalties (the publisher commission most of their books on a flat fee basis), but I estimate that in total, including the fee, I've made three thousand pounds from this book. This extra two grand came from articles I sold to magazines and newspapers on the back of the book.
I was paid an advance of 500 pounds for my second book and given royalties. The book took months to write and was very stressful. The topic gave me nightmares.
So far, I estimate I have made round about 4 thousand pounds from the book. I've had to do a lot to promote it and a huge chunk of that money has come from the PLR and ALCS.
Without that money the reward for so much work would be an insult. Well, at an income of four thousand, I estimate I made less than a pound an hour. Only a writer would work for that!
So, what would my advice be -
If you want a guaranteed fee, opt for the flat fee. But, only if it's FAIR.
I would still always try and get royalties and an advance. Your book might sell well, like my flat fee one did. But, remember, with royalties you will be relying on the publisher to actually sell your book. Of course, you need to be proactive there too.