Cost is a matter of negotiation between an individual freelance and a client. However, the SfEP suggests minimum hourly rates for various editorial services, and the National Union of Journalists' freelance fees guide lists similar rates, plus others for writing, design, translation, etc.
Although these rates are only suggestions and not enforceable, we encourage members, associates and others working in the profession not to accept lower rates. You will be able to find individuals who will do editorial work for less than these rates, but they may be untrained or inexperienced or both. The dictum 'Let the buyer beware' is as true for editorial services as it is for any other purchase.
Many freelances prefer to negotiate a flat fee for an entire job. This has the advantage of allowing the client to budget for work more exactly, and it is also a method preferred by Her Majesty's Revenue and Customs (HMRC). However, it can be difficult for a freelance editor or proofreader to give a realistic estimate of how long a job might take or how difficult it might be, especially if they have not had sight of the material before making the estimate. Therefore, it should be possible to renegotiate a fee up to a certain point in the work – say, when a third has been edited/proofread – by which time any potential difficulties should have been discovered.

I am not a professional proofreader, so I don’t have first-hand advice to share. However, a quick Internet search revealed a variety of rates offered by professional proofreaders. Some include:

Per hour. The Editorial Freelancers Association lists common editorial rates of $30 to $35 per hour for proofreading, based on a proofreader’s ability to cover 5 to 10 manuscript pages an hour.
Per project. According to  answers that LinkedIn members left to the question “How much would you pay for a good copy editor,” proofreaders and copy editors often charge by the project (more on that in a minute).
Per page, with a page equaling approximately 250 words. Compass Rose Horizons, an editing services company, charges $2 to $12 per page depending on how much proofreading, copyediting or rewriting is involved.
To that, I would add that the price you’d pay a proofreader, or that a proof reader would charge, would depend on the type of copy that’s being edited or proofed. It would also depend on the standard by which the copy would be measured – i.e., does it need to be AP style or something else? – and if the proofreader would be expected to do some amount of rewriting or just go over copy for grammar, spelling, fact checking, etc.

How Proofreading Rates are Determined
While each company sets its own rates for proofreading jobs, generally companies offer clients a variety of rate schedules depending on the difficulty of the task and the length of the material to be proofread. Some companies and individuals charge their clients for the hours worked on the project; depending on the proficiency and speed of the proofreader, this may be far more expensive than projects charged by the page or by the word. One advantage of per-word or per-page proofreading rates is predictability; companies know beforehand exactly how much their proofreading project will cost and can plan accordingly.

The Proofreading Girl strives to provide professional proofreading services at an affordable cost to all. Below are the base rates for most proofreading jobs. To get an exact quotation for your specific project, please upload your document.

Rates are tiered according to turnaround requirement. Same-day service carries a premium, but for less urgent projects, a longer turnaround may be more economical:

Gold (24-hour turnaround): $9 per page
Silver (48-hour turnaround): $7 per page
Bronze (standard turnaround): $5 per page

Distributed Proofreaders Project