The rate you should charge to proofread a translation will depend on several factors. First, the complexity and length of the translation will be taken into account, since a longer and more complex translation will require more proofreading. Additionally, the proficiency of the translator should be considered – a novice translator would require more editing than one who is more experienced.
You should also assess your own expertise levels. Your rate should reflect the number of years of experience you have serving as a translator, proofreader, and/or editor. Lastly, the type of translation should be taken into consideration — for example, medical translations might require more attention than conversations.
Ultimately, it is up to you to determine an appropriate rate for your proofreading services. However, you should do your research to ensure you are competitively priced and maximizing the value of your services.