http://www.aaa3.net/how-to-scale-for-drawings/

How much to charge for a mural??
A man wants me to paint a portrait of his daughter on the wall above her bed. Now, I know this may not necessarily constitute as a mural, but I’d like to use your suggestions for future reference. I don’t think it will be too hard… probably around medium on a difficulty scale. And I’ll probably charge based on size… but how much on that rate?
I don’t have a lot of experience, (did two murals before) but I have done several paintings/drawings. I have a lot of talent, and people have always been VERY impressed with the results.
I want a good price for both parties involved; I don’t want to overcharge the guy, but I don’t want to shortchange myself, either.
I’ve done a couple or murals and I just charge for materials and then by the hour. (go buy some paint and hand them the receipt)
Sounds like you have some experience. Maybe charge $15- $20 an hour.
Scale Drawings