Some people get confused about the difference between editing and proofreading, but in reality, the two are quite different. Proofreading is the last, final check before your novel is submitted to an agent or publisher.
So, if your novel has already been edited, maybe by a friend/family member or another consultancy, you just need a professional to simply read it and point out any remaining minor typographical errors - that's proofreading.
So, proofreading is the final check of a manuscript once all of the other editors have finished their edits. Once the proofreader has finished his/her task, if no further errors are identified, the manuscript is ready to go to print or to be sent to an agent/publisher if the traditional model is being pursued. The proofread should not, in theory, find too many remaining mistakes, as the vast majority should have been picked up and corrected by this stage, although in practice, it is very rare for a proofread to find no errors at all.
On some occasions, largely dependent upon the quality of the previous edits, the proofread can reveal a great many errors, and in some cases can even suggest that a complete revision of a manuscript is necessary – this most often happens when an author has skipped the previous edits or done them his/herself to save on costs.
In any case, a proofread is a necessity before either self-publishing or sending a manuscript off to an agent, as most agents will simply reject a manuscript as a matter of course if the initial chapter is riddled with errors.
As this is very similar to line and copy editing and usually carried out by the same grade of consultant (though importantly, never actually the same consultant who has done the editing – based on the fresh eyes principle), it carries the same price tag of £30 - £40 per hour depending on the complexity of the manuscript and experience of the proofreader. As a general guide and for example, a young adult fiction title would be cheaper to have proofread than a non-fiction book on metaphysics, simply because a specialist proofreader would be needed in the case of the latter.