Exact rates will vary. Among the factors which can affect rates are: how well written the text already is; how well presented it is; how much work you require beyond simply proofreading the text (indexes, for example, are complicated things and attract a higher rate) … and of course how long the text is!
I am a member of the Society for Editors and Proofreaders, and you can see their suggested minimum rates here – currently set at £24.30 per hour for proofreading. That minimum is based on the assumption that the text is in decent condition and doesn’t require a lot of heavy work.
As a rule of thumb, and based on the same assumption, one hour of proofreading work is equivalent to roughly 2,200 words.
I accept payment via bank transfer (to a UK account) and PayPal.