Rates do not just happen, as we all know (but may need reminding): you need to set them, set them well and keep pushing them up as you grow professionally. Translation agencies are surely not the best you can do when it comes to charity, so you should charge them as much as you can.
From the Patenttranslator's Blog, 10 Signs That Your Rate Per Translated Word May Be Too Low
From Thoughts On Translation, What is "the right rate" for your translation services?
And, while I am at it, here is another post I should have passed on weeks ago and did not. I thought and wrote about this at some length a few months ago, but the Patenttranslator's Blog clearly knows best: Is 50 Thousand Dollars Money Well Spent on a Translation Degree?