Imagine a translation so good that you only need to read it, and perhaps mark a change or two, or even fewer, per page.
Now imagine a translation so poor that parts of it are unintelligible, and the parts that you do understand are only comprehensible because of your knowledge of the source language.
Clearly the difference in time required for these two cases will vary by a factor of 10 or even more.
Most editing jobs are somewhere in between these two cases, but both of these extremes are realistic scenarios.
The best way to price editing is, therefore, by the hour. If the client needs to know your quote for the entire job before you start, calculate it on the basis of some sample passages. Be sure to review the whole document before making your estimate, to ensure that your samples are representative.
How do you know how much to charge per hour of your time? Base it on your own per-word rate. How many words can you translate per hour in an average text (mid-way between the hardest and easiest texts you typically translate)? Don't forget to account for time spent on research and proofreading.
To help you get an accurate idea of what to bill, try tracking the total time spent for a few of your typical translation projects. Divide the total earned by the exact time spent translating, researching and proofreading to find out how much you earned per hour of work.