I generally do not use one, I have drawn them up from time to time detailing the scope of the job and what the owner's part would be once my job is complete on complex jobs but generally speaking, I don't use them.
There are usually hidden variables on bee removals so its good to give a high price and tell them or less, for when the job is easier than you thought.
If you give a price of 3-400.00, they will usually be fixated on the lower price.
If I want 400 for the job at its worst case scenario, I tell them to expect to pay 400 but if the job is less difficult, I will reduce the cost.
Knowing what to charge comes with experience but even still, there may be an occasional job that will surprise you.
Ask questions, ask questions, ask questions, then investigate the construction, really look the bldg over. If you are uncertain of what to do consult with a contractor, take pictures.
Don't rush your decision, really think it through. Some you will make out fine, some you'll wish you charged double, it will even out with experience.
Be honest and fair with yourself and the customer. If the job is much more involved because of a certain aspect of the construction, explain this to the customer, look them in the eye and ask for more because of this curve ball. Convince them why you need more for the job.
I rarely have to increase my price but when I do its because of some construction flaw that allowed the bees to enter a space that if the area was done correctly, they shouldn't have been there.
Most people are understanding and will appreciate a direct and honest approach, sell them on yourself first and everything else falls right into place.