Generally, I take my guidance from the CIEP suggested minimum freelance rates for proofreading and copy-editing (for the current rates please see the link given below) but I'm happy to tailor my fee to the individual client, their circumstances, and the extent and time scale of the project.
If you're a company I might ask you to provide me with a representative sample of your work (no more than 1000 to 1500 words), so that I may better estimate the length of time it will take me to complete the project and hence the cost to you.
