A hand painted mural would be highly subjective to how long it will take you to do. I know true mural painters are artists, and charge accordingly. I would suggest figuring out how long it will take you to do from start to finish, and charge by your hourly rate, +materials, +profit, etc...
I think the price should be guaged on more than just time. Murals that have been done multiple times are easier and therefor faster. I think size and amount of detail are two factors to decide price on. Of course time and materials always comes into play.
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