Hiring an editor for your manuscript or your indie book can be very expensive. Benefit : A well-polished book. Truth be told, no matter how many times you self-edit, you will always miss something that can be corrected, a paragraph that would communicate a little better if it were rearranged, etc. Drawback : You may not see the returns on your editing investment. You probably won't get away from having to spend at least, and I'm seriously giving you a low number here, US$300 to get an editor to comb through your entire manuscript and fix more than just your trouble with subject-verb agreement, "Whenever he say it, I always laugh at him." So, is it worth the investment?
If you take your writing seriously and want to be taken seriously, it is. You can have a pretty bad start off as an indie author if people see that the quality of your work isn't up to par. But on the same token, you have bestselling greats like indie paranormal romance writer Amanda Hocking who admittedly--last I remember--does not have an editor, and while some readers certainly do complain about that when they read her work, she has a loyal, awesome following. She's tearing up amazon charts, to put it lightly. I don't even know the impressive figures, but google her and you'll find out. She is selling more ebooks and books than most published authors with a team behind them can pull out. If that doesn't scream awesomeness, I'm not sure what does.
If you can afford an editor, it stands to reason then that you must also put your money to good use and market the heck out of your book once it's done. Don't write the book, pay a bunch for editing, put it somewhere and say, "Hey! My book's available. Read it," and expect that it's gonna sell itself. If you've invested the money and energy in preparing the book, you'll want to invest the same amount of energy and heart into letting people know why they should read it.
If you can't afford an editor, you can ask a schoolteacher, a professor, or even an author friend who has been published. Many people get beta readers, several people who comb through your book after it's written and point out flaws and mistakes. Do yourself a favour and at least don't rely on self to get editing done. Another pair of eyes is important. Ironically, having an editor does not equate to people automatically liking your book, automatically buying your book, but one would argue it does up the chances. And really, if it's your work, and you love that piece of work, you'll want it to reach the public in the best state that you perceive it can be in. I myself hired an editor for my debut novel "The Mirrors of Fate: Out of the Past" which you can check out here.
But, you interject, "I've combed through my book enough! What did I possibly miss?" I combed through my debut novel more times than I can count, but my editor still helped me pinpoint, correct and fine-tune. Having lived in the US and the Caribbean, I hop all the time from US English writing style to British English writing style, so my editor helped me keep it US English. A sentence that makes sense to you might not always communicate as effectively. It was worth it to really get that polishing in. Yes, it can be pricey, but if you can afford it, go for it with the knowledge in mind that you can't let up on the marketing side.
So, indie authors, go forth. Find your best route for ensuring that your book is the best that it can be before it reaches readers.