The artist most likely will require a down payment for the concept.
Once he/she starts the art work it will either be or a combination of the two:
Cost of supplies and time, hourly and by the piece (is it a large piece, small piece, does it require painting from a ladder or a scafold?)
Or the artist may just charge you for the piece but if he/she does that then you should tip REALLY, REALLY well.
If you are ALSO hiring an artist to paint an exterior wall that faces the public I highly suggest tipping in the hood of 50% as to keep your wall clean and safe from taggers.
If you are hiring someone from that community and you piss them off then they are most likely to come back and piss you off.
Now, if you hire an artist and he/she sucks (can’t go from paper to mural)
Then you should hire an artist from outside of the city to come fix it. Not someone in the circle.