£17 per hour proofreading and copy-editing.
Special rate for students £7.50 per 1,000 words. However, this is based on a good standard of English, probably native. Non-native English inevitably requires a lot more work and on the supply of a sample the price quoted will take this into account. As the checking of references is usually very time-consuming, this will be charged at the rate of £17 per hour.
It can be difficult to estimate exactly how long something is going to take. Using professional judgement I am often happy to provide a fixed price for a job based on an approximation of the time I think it will take to complete.