My wife did some freelance editing a few years back and $50/hour was her going rate. She might have gone lower some for friends and regular clients, but she insisted on an hourly rate instead of per-word rate because some manuscripts were more of a sh*t show than others. Also have to make clear what the "editing" is up front -- just typos and sentence structure, or a full edit including fact-checking?
I edited a book once for a friend who ran a golf course, I negotiated a set of new Ping irons as my fee. Turned out to be a lot more work than I thought, to the friend's credit he asked if the deal was fair. I lied and said it was, though I did hit him up for a new 3-wood too.