1. If you don't know what kind of editor you need, K.K. Rusch breaks down the classifications here: http://kriswrites.com/2013/01/30/the-business-rusch-hiring-editors/
2. Once you've figured out what kind of editor you want, it's on to cost estimates. More than a car, less than orthodontia for your mini baseball team? Here's a starting point courtesy of the Editorial Freelancers Association: http://www.the-efa.org/res/rates.php
3. After you've sold your kidney and half a liver, in whom do you entrust your book? You want someone with experience in your genre, sub-genre is even better. Then you want someone who is willing to do a "free test" of X number of pages to see if they want to deal with your work and if you can deal with theirs. For what do you need to beware? The SFWA breaks it down here: http://www.sfwa.org/for-authors/writer-beware/editors/#What
4. Where do you begin to find a good editor worth his/her fee? That's a lot harder. Networking is the best answer. If you're just looking for a reference list as a leaping-off point then there is the invaluable Predators & Editors: http://pred-ed.com/peesla.htm
If you're hunting using a personality-check filter, then I offer up this slightly outdated twitter listing courtesy of Galley Cat: http://www.mediabistro.com/galleycat/the-best-book-editors-on-twitter_b11241
So, dear reader, do you have a great editorial resource? Share, please? Pretty, pretty please?