It's not an odd question.
Every business should know its costs, right down to the last penny, so that at the very least you can calculate you breakeven point, which is a value of sales your business needs to reach every week in order to stay afloat. (breakeven is the point where you have neither made nor lost money)
Fuel and chemical costs are the two main costs that you only incur when you go out to work and are called 'variable costs because they vary to the amount of work you do, usually about 8% fuel, 4% chemicals, so a variable cost of 12%.
The rest of your costs are call fixed expenses, these cost occur whether you are working or not - these are the ones that catch people out.
I have a full financial planning system on an Excel spreadsheet, you can have a cope if you like.
Hope this helps
Simon