How much should I charge?

I found this website that advertises that they will write your research papers for you for a fee of course. I proceeded to write to them and tell them all the grammatical errors I found just in the first two paragraphs of their website, and that I wouldn't use their services if I lost use of both of my arms. I suggested they hire a proofreader next time to go through their website, because they needed to make a good first impression if they hoped to get any business. So they write me back and ask if I would do the proofreading for them, and they'd pay me a fee. What should I charge?