How much to charge?

You have to decide weather or not to charge by the hour, per piece, or in payments on the whole. Start with deciding how much you think your worth per hour. Keep in mind they are paying you mostly for knowing where to swing the hammer and some for actually swinging it. So you must calculate all the years you put in to mastering you craft into your hourly rate. Picasso would charge $10,000/hr for instance.
Once you have this you can charge by the piece, just guesstimate about how many hours it would really take you to do it and double it. This is not to cheat your clients but to cover your arse for when the client gets change happy. You can also estimate the cost for the whole project and take a half or quarter up front depending on how soon you can start showing them something. You'll have a hard time getting a final payment on something that took 3 months for a first draft.