As an editor who has worked for a number of digital presses, royalty only payment, while common, is not good if the goal is to earn money.
I've worked for a few places that paid a flat rate of $200 per book and others who have a set fee per book but will pay part in cash upon completion of editing and the other part at release and those who pay part in cash at completion of editing and the rest in royalties up to a certain amount and those who pay only in royalties. I know I'm preaching to the choir here when I say that for all the hours spent on those books (and I NEVER sacrifice my quality standards), I made pennies. When I did that job, I was new but it didn't take me long to realize my effort, time and talent are worth more. The other payment plans aren't great but the presses aren't New York either as far as money, and I'm okay with that. It's at least something until I decide if I want to do this full time (I have a fabulous day job) and make the leap to New York or big commercial presses.
Over the years, I've worked with editors who most definitely do not work as hard for royalties than they would if paid a flat fee or even partial. Or if they start out strong, when they start seeing what is actually coming in, they either leave if they are good or stay. He thinks he's going to make $15,000 per book? Did I really read that? In the past six years in all the houses I've worked with, only a handful have come close and over. Guess how much the editor saw? $150.00 ($75 up front and up to $75 in royalties).
I'm sure editors will respond. Likely they will be inexperienced/new, use that editing experience to get some work under their belts then go off and offer services as freelance editor (I know several who did just that) or move on to bigger, better paying houses.
It's a risk publishers who only pay royalties have to take. That business plan usually results in high turnover with editors or the editors who stay may be serviceable but not ones who have high quality standards. Now, before someone beats me up , I fully recognize, as I know a number of these types of editors as well, there are reasons for staying such as they love what they are doing and don't need to worry about the money and are happy with what they get. They can be strong editors and perfectly content. I know some of them too.
However, for someone like me, who wells knows their value, I'll never take another position for royalties only. My work and time means much more to me than that.
An Old Hack, if an editor is not renewed or quits, in most of the presses they get nada. No more money. I've only heard of one press, and the name escapes me, that continues to pay editors royalties when they no longer work for the house. Most quite clearly state in agreement no more payments if editor quits or contract not renewed.
I wish Travis luck in finding (and keeping) good editors.