I am open to outdoors murals on walls, interior wall murals and even custom paintings. In general, I take 20% down payment, 40% upon job start and 40% upon job completion by contract only. Each contract is for a set reasonable time frame that will cover the standard time say 2 weeks for small job or 4 weeks on a large job. After that time, any extra time or changes are at $50/hour and after last completion payment already made. All materials: paint, brushes, spray cans, tarps and other equipment are paid up front before the start of a project and property of the artist there after. Generally, its $0.75 a square inch so a 4 feet x 8 feet mural would be $3,456 + cost of materials. I may be open to discounting for say public murals or art gallery walls as it promotes me as well. Below is a standard commission contract I use to browse over.
Time: 2-4 weeks
Costs: $0.75/sq inch + materials used limited + overtime is $50/hr
Buy Materials upfront + 20% down payment
Start Painting Day = 40% payment
Last Day = 40% payment
Overtime (if needed) after final payment per day.