This question has been asked a few times on this forum, and the best answer has always been this in my opinion:
How much is your time worth?
Because you can charge whatever you like, so long as the client is willing to foot the bill. It's up to you how much you charge, often when people are starting out they charge a lot cheaper because they are new to it and want to ease themselves in.
If you are confident about your skills, then charge the maximum you think you should get paid.