No one fixed price can apply to everyones situation and area. One of the simplest things you can try is find a fence which you can conduct a few test runs on. Clean one section and make note of the time which it took to complete. After you have that figure, run some numbers on what the square footage of the single section is so you can determine how much sq ft you can wash in 1 hour. Run those numbers against how much you need to make in a given hour. with a little division, you can determine how much to charge per sq ft. to match or exceed what you need to make per hour.
Mike