Fixed costs include rent, property taxes and cost of capital. Cost of capital is money you have tied up in the business, such as tools, equipment, office furniture and computers. If the money was borrowed, your cost is the interest on the loan. If it is your money, you are giving up any alternative income it could be earning invested elsewhere. Fixed costs are usually recovered with gross margins.
Variable costs increase as work is done.
Variable costs include labor, utilities and body materials. Variable costs are usually recovered through a charge per labor hour and also contribute to your gross margins. Know your costs. Know which operations cost you how much. Know how your sales recover each cost and contribute to your gross margins.