What would a contractor charge to drywall ,finish and paint a 20by 9 wall,including materials?

If you are able to figure out the material costs yourself, the contractors fee will be equal to the cost of the materials, usually. This will depend on the cost and thickness (type) of sheetrock you are installing, mostly, as thicknesses range from 1/2 inch to 5/8 and the outer surfaces include the normal fibered paper and a water resistant type known as "greenboard" for use in bathrooms.