I agree with Aaron, seeing as it is your first freelance offer I'd advise keeping the charge low. Even if you feel you're undervaluing your skills it’s still best to start off cheaper and get your foot in the door than never get it in at all. Once you have had a couple of clients, and quite possibly received further work from them, that’s a good time to gradually increase your rate. If people are coming back to you then they aren’t just happy with the price, they are happy with your level of work and what you can produce for them.
Best,
Iain -
http://www.beakable.com