Editing is pretty tricky to put a price tag on. Depending on the kind of editing you want to offer prices can and should vary. Developmental and environmental editing, aka. plotting out a story, lore, or worldbuilding is as much editing as being a human spellchecker.
If it's just grammar and spellchecking, you can either do a flat price per chapter, a per-word price, or a combination of both. Think about what you can offer to enhance the experience and then about how shameless you want to be about it. Professional editing prices are insultingly high since the market is pretty much free, anyone can call themselves an editor. I've seen people successfully charge 25$ for spellchecking an A4 page...
I translate and edit professionally for games, and I always include the editing fee in my translation since doing bad work leaves a bad aftertaste. For a clean translation, I'd charge 3ct or 4ct editing fees per word, depending on how complex the text and editing are.
For novels, I set the prices lower, since it isn't my main gig. I charge up to 2,5ct per word and almost always round down a few bucks to get clean round numbers. But that's my problem of being too generous. Up to 5ct per word would still be on the cheaper side of "professional" editing.