I take my hourly rate (or man hour rate) and multiply it by the number of hours it will take me to do the job. If the job is going to be complicated it takes longer, so its already in the bid.
I will add some to the total if the home/buisness owner looks like they are going to be difficult to deal with or will be constantly bugging me. BEWARE of retired old ladies!!
I will also add some if the job is on the other side of town which=more driving= more gas and time. Also, if i need to rent or contract other help (crane, skid steer, ect) it gets added to the job cost.
My hourly rate varies depending on how many people are working, I bid 41.66 dollars a man hour, or 125 an hour for a 3 man crew. As you develop more of an overhead you should be able to do more per hour, so your hourly rate should go up, but not the total price of the job (with all other variables being the same) because you will be in and out faster.