It all depends on whether it's (merely) a proofread, a line edit, a copy edit, or whatever. Of course a mere proofread should be cheap, in the 0.1 to 0.3c/word range ($100-300 for a 100K novel).
Many good proofreaders add more than proofreading--in essence beta reading and giving feedback. How much and of what kind and whether it's what you want is always negotiable along with the price.
Thus, there's really no single "right answer" when it comes to proofreading/editing/feedback. For example, for certain friends and certain other pro sci-fi authors I do edits, and I charge a minimum of 1c/word, often more. I don't try to give a book definition of what this means, but for that price they get a great proofread, plus lots of feedback on everything that occurs to me--plot holes, characterization issues, word choices, awkward bits, whatever. I've never had anyone tell me they overpaid, yet I'm not doing one of these for anything less than $1K. Even then, in the strictest sense I'm probably losing money--but I do it now and again to maintain relationships and help out friends and colleagues.
I only point this out to illustrate that you can't tell merely by price. It's value for money that matters, and that's by agreement between the provider and customer.