Copywriting can cost anywhere from less than $10/hour if outsourced to Asian countries such as Malaysia, India or the Philippines, to $160/hour for a very capable, local (Australian) copywriter.
As you seem to have already worked out, the difference in payment tends to revolve around the work done to research, edit, optimise, and even test and revise the copy, in addition to "merely" writing it.
At the lower end, I would expect reasonablly well-written copy, but I would need to provide a detailed brief, usually with my own research, and I have to edit, and test the copy myself. Often, this work means I'd prefer to just write it myself anyway. But if I want a piece of truly great copy, written by someone who is a genuine professional, especially if it is a formal piece of work such as a whitepaper, I have no hesitation paying higher rates.