The problem is that there's a lot of predators trying to take advantage of newbie and wannabie authors, desperate to succeed. Therefore it's wisest to follow Yog's Law: "
Money Flows Toward the Writer".
That way you at least know you're not getting screwed. Serious publishers do not ask the authors for money and will offer the editing for free, for a share of the future profits.
Of course there exist honest editors out there who don't make a habit of taking advantage of anyone, but for a new author on the market, you have no way of knowing. Best to stick with the law.
http://www.sff.net/people/yog/