I buy my material on line and can get an good price for material. Trust me materials are minimal cost compared to your personal time. I recently heard a recommendation that I like. Find a comparable competitive artist at your skill level and research their prices in the same market you are in. Then calculate the cost of their artwork by the square inch. That way you can determine how to price your artwork by size. Establish a high , medium and low price range and determine where you want to be based on your skill level and market.