Here's a little story...
About eight years ago I searched for freelance editors online. I whittled the choices down to five from the dozens I researched. Each one asked for samples, anywhere from 5 pages to 10. I sent each one what they requested.
All five caught the simple typo's, spelling and punctuation mistakes, but each one had a different style and noted different things about the samples. Voice, PoV shifts, paragraph formatting, sentence structures, and on and on the list went.
One editor actually tried to inject purple prose into my story with her sample. I later learned she used to edit Elf Quest magazine, I think it was, or something similar. She would not have been a good fit for me...
All wanted to charge me between 3 and 5 cents a word. Only one editor had the balls to tell me I was not ready for them yet, but he would be glad to take my money. If I ever went back, he would have been my choice...
The dollar a page noted earlier is generally for short works like a query letter, synopsis, business letters and such. Not for novels of 60,000 words or less.
Also, Line item editing and Proof editing are different so they will have different prices. What you do is talk to the editor on what you need and what they can do for the price you can afford. Note...the more you have them do, the higher the price.
Personally, I can't afford the price and have spent years learning to edit my own work. There is a very good thread her on what freelance editors do in the ask the editor forum, I think. I would suggest you read it before choosing an editor as it is very insightful.
Good luck...
PS, my 140,000 word novel would have cost me over $5,000. for full editing and proof reading and no guarantee that my book would have been picked up by an agent or publisher. There is also the fact that if a publisher did pick it up, their editor would have gone through it as well. Any editor that guarantees you their services will help sell to a publisher is not an editor I would use.