Brian's advice is dead on. Most states and counties have a small business economic development office. These offices usually have seminars or classes about starting a business. The costs of operation to consider are many. In addition to the usual overhead costs (rent, utilities, etc) there are also the non-obvious costs like business license, any EPA or environmental compliance costs, the cost of your accountant, legal advice, insurance, etc. Don't forget the investment in tools and equipment and depreciation of that equipment. There are consumables (parts cleaner, hand cleaner, paper towels, etc.). There are disposal costs for "hazardous waste" - which is just about everything these days. There's certification costs.
You need to start by understanding exactly what costs will be incurred over a year, then figuring out how you plan to allocate these costs to a particular job.
Oh, then there's always the issue of how much you pay yourself.