This is a difficult question to answer without knowing more about the project in question and your own qualifications and experience. That said, here are a few general guidelines that might help you to come up with a rate:
Consider charging by the hour, rather than by the project. This will give you more flexibility to adjust your rate according to the particular project's needs.
As a guide, average rates for copyediting in the UK tend to fall between £25 and £30 per hour.
When setting your rate, be sure to factor in the type of copyediting required (e.g. developmental, line, or proofreading), the length and complexity of the document, the deadline, and your own experience and qualifications.
You might also find it helpful to look at this article on copyediting rates, which provides some further information and examples:
https://www.the-efa.org/resources/rates/