besides just the costs of tools and certification, there's the lease cost of the shop itself, insurance costs, and business taxes, including annual personal property taxes on teh tools you already own, based on their value. There's the cost of the credit card machine so you can take credit cards. There's the cost of business phone systems. There's the cost of phone book ads, and regular advertising. If you have employees, your insurance goes up, there are the costs of employment taxes, payroll taxes, functional facilities for their use, and more. An employee that you pay $10 an hour will cost you twice that.
Cleaning materials and waste disposal fees are included in the overhead costs, as well. And we haven't even gotten to inventory of frequently used materials.
It seems that most small businesses have to "cheat" the system in some fashion just to keep their doors open.