I tried using one once with a short story (Edit: ~6k words) just to see if they were good and if I'd like to get one for my longer projects. To be honest, I wasn't that happy overall.
I decided on a small bureau that opened close to where I live but we worked over email. I sent them the document I wanted edited and they sent it back with comments.
Other than one stylistic choice where we disagreed, the rest were small stuff like words or images they didn't deem fitting (e.g. "childhood" since the story was set in a middle-ages inspired fantasy world or "brotherly love" because real brothers aren't always close ~). I changed more than half of those small things but kept the stylistic choice (It was about having a storyteller which they thought wasn't something that should be done anymore in this day and age while I thought it fit the narrative pretty well and since the thing was inspired by Arthurian legends ... well. We didn't agree there.)
The whole thing cost me about 100€ and I honestly wouldn't do it again. The things I would have been interested in (and that I paid for according to their price list!) would have been characters, setting, plot - that kind of thing. And I got zero comments in regard to any of that so it was more or less a waste of money to me. But at the very least I know who I wouldn't hire anymore
Edit: I'd just like to add that there are different types of editors. Depending on which you want the prices and the feedback should change.