Everyone has hit the high points - fuel, maintenance, insurance, wages and repairs but have left out one of the largest operating costs - the cost of the machine. A hunk of yellow/orange/white scrap iron costs money to purchase, whether you pay cash or finance and you need to get a return on that investment of more than just a good wage for yourself.
Even if your machine is paid for and you don't have a monthly "love" note in the mailbox, this hunk of scrap iron depreciates every minute of every day, so you need to also include that as an operating cost as well (even if you own it "out right" or it's financed).
Oh BTW, we didn't touch the subject of "office" overhead, business licenses, taxes, etc. Even if you do your own billing, accounting and tax preparation, you still have time, a lot of time, involved in those tasks as well, that need to be included in your operating cost.