Editing is a fickle business. There are editors living in 3rd world countries that can live very well on $10 a day, who can afford to charge a very low rate. Other editors live in nice houses in expensive cities and need to charge more. Some editors are university graduates, some have PhD's. Some are very good, and some aren't.
As an editor - how much should you charge? As a writer, how much should you pay? Unfortunately there is no standard 'going rate' for editing; editors have unique styles and services. Prices can depend on level and depth of editing, on turnaround speed and other variables. Here are a few tips to make sure you're charging/paying the right amount.
If you're a freelance editor, you need to find a way to charge fairly and consistently. Your price has to reflect your quality and skill level, but also be competitive. A good strategy: Search through about 10 editing sites that you like and that are offering similar services; ie your competitors. Look at how they charge and how much - and then shoot for an average. If the cheapest is $10 an hour and the most expensive is $50, settle on about $25. Although everybody is searching for a bargain and a good deal, when it comes to editing people are already afraid that the service won't be good enough. They are paying for editing because they NEED their writing to be great, flawless, perfect. Therefore price is rarely the determining factor. They want the best editor, most experienced and most suited to their project. So as long as your price is reasonable, you're probably OK.
You should also look at how much you'd like to make per hour. There are many ways to charge clients - personally I prefer by word (or page, although that can be less precise) because it lets visitors check exactly how much their project will cost right away. But figure out how many words you can edit in an hour, and based on that, how much you need to charge per word. I think $15 an hour is a pretty reasonable rate for editing - although some editors make much more. Therefore - if I can edit 1000 words an hour, I could charge $.015 per word.
Now that you've gained these peaks into the business of editing, you writers should have a better idea of what to expect. Choosing a qualified editor can be difficult. Don't assume that the most expensive is the best, and also don't assume the the cheapest is untrustworthy. Read the editing services site, read the testimonials and see if they sound genuine. If it's possible (and it should be!) send them a 1 page sample and see how well they do.
Compare a few sites and get some price quotes - then you can set a reasonable budget for yourself; it should be a price that you feel comfortable with and that you think should be enough to get a qualified editor. If you want a bargain - email some of the more expensive sites and ask them if they will drop their price... many of them will.
You can also search freelance sites like guru.com or elance.com and get freelance editors.