Oil paint is more expensive than acrylic, watercolor, or gouache paint. Since it is not water-soluble, you need to use painting agents to dilute oil paint. Oil paintings need to be primed with special primers as well as sealed after the painting is done. All of this adds to the material cost of oil paintings. These paintings also take longer to make as it takes longer to dry and therefore the labor/hourly rate of an oil painter will be higher than that of most acrylic painters, for example.