If english is your mother tongue than proofreading will be cheaper for you:) Some people charge per hours, some per number of words, so in your case it would be cheaper to have someone charging per hour. Rates are very varied, I have heard from £200-500 per thesis. A suggestion from my experience-choose somebody who has been recommended to you by someone who has used that person's service. Otherwise it will be like with me- I paid for proofreading but both examiners still pointed out many grammar mistakes in thesis:(((((