How much will it cost? That depends. Every job is different.
If your text is in great shape and needs a quick final proofread for typos and grammatical errors I am likely to be able to read more than 3000 words in an hour. For material which needs more intensive work I might read less than 1000 words per hour. Other tasks, such as checking a bibliography, are time intensive and the number of words is not the only factor when estimating the time it will take to complete the job.
You might need work done urgently and be willing to pay for me to work outside my normal hours. You might be booking my time several months ahead so that I block that time specifically for you.
All these variables mean I don't have a fixed price per word. What I will do is look at a sample of your text (eg book extract or journal article) so that I can judge how much work is necessary and how long it will take.