This is an issue that I'm deeply torn about.
I'm certain my work could benefit from editing. But editing is a skill that does not come cheap, and considering what it entails that is very understandable. I didn't realize I was going to go this route when I started writing my book. If I had, and I had done my research then, I likely wouldn't be banging my head against my desk over this.
But my novel -- which needs another round of edits from *me* still -- is in the 130,000 word range and the average cost I'm seeing when I look stuff up on Google ranges from $2 - $4 a page (which for the sake of simplicity we'll assume would be 250-300 words). That adds up really quick.
On one hand, I've had people tell me not to beat myself up over it. It's not a question of whether I *care*, but rather whether I can *afford* this. On the other hand, I feel like I have invested in everything to do with this project but the thing that should matter the absolute most: making sure that what I am presenting to the world is the absolute best that it can be. Editing cannot make something that is crap become amazing. But I strongly believe that another set of eyes (or more; as has been pointed out there are different types of editing) can take something that is already "good" and make it, if not great, then at least "better".
I'm not worried about selling a million copies. I know with or without editing that some people will like what I write and some people will despise what I write. But I also know that I have a great story idea and that my characters deserve every advantage I can give them. I feel like I am cheating them, in a sense.
Should
you hire an editor? That's something you need to come to terms with for yourself, and if what I'm saying above is any indication, it's not an easy decision. My apologies for turning this around to an extent and making it about me, but I am still in the process of getting my book ready to be published and it seems I am questioning the same thing you are, in a way. I wish I could have been of more assistance.
Good luck!