I am always harping on people who self-publish to edit their work. I am an editor, but I am not saying you should have your work edited just because I want you to hire me. I am saying it because most people are not going to recommend a book that they struggled their way through because of editing errors. Generally, every time I publish a post on editing, it is because I recently read a self-published book that was awful because of the editing.
Today, I was reading a book that was self-published through a vanity press. In case you weren't aware, a vanity press is a brick and mortar press that will print your books for you and occasionally help you market and distribute it. However, unlike CreateSpace, they charge an arm and a leg and make you purchase a certain amount of hard copies when you do it. Before the internet, this was the only way you could self-publish - but it isn't really self-publishing. It is also not what I call "assisted self-publishing" - an online publishing company that charges you to print your book, but turns around and publishes it for you with CreateSpace/Kindle (or through another online self-publishing company).
In the old days, vanity presses always edited your book for you - but you would have to submit an order of thousands of books at a time. They were also fly-by-night and occasionally authors would pay to have their book published and never receive the hard copies because the company would go under before they finished. Now, with print-on-demand (through modern technology), they still cost an arm and a leg, but you can choose from a variety of packages and you can order fewer books. It will still run you thousands of dollars to do it, though.
Sadly, editing services are often offered only in the more expensive packages. Looking at their prices, I know you can find freelancers to edit your work for less. So, I did some research, and I looked at how much freelance editors will charge you for their services. As expected there is great variation in rates charged, but in general you can expect to pay $2.50 - $12.00 per page (double spaced - 250 words) ($0.01 - $0.05/ word). Rates vary based on what type of service you need - obviously content editing costs more because the editor is making your manuscript flow and fixing problems with content that go beyond standard grammar and spelling errors. Paying more money does not necessarily mean you will get a better editor.
If you post a project on Guru (or elance), look at work samples of the freelancers who bid on your project. Make sure the person is (1) finding errors and (2) not correcting something that is correct. Generally, editors on freelance websites are going to charge on the lower end of the spectrum because they are bidding against others, but be careful of choosing someone simply because they bid lowest (or highest). If you don't want to hire a freelancer, you can also get Grammarly to edit for you, but I think you have to have a subscription and actually put the document into the checker before you get that option.