Close, but on a flat bed the head ache rack is the aluminum (usually) behind the cab. It keeps unauthorized cargo from entering the cab in the event of an emergency, thus preventing a head ache.
First off, I can see from what the guy wants that he is going to be more demanding than he is letting on. If it is just one truck, I would not do it for less than $100, and that might be too cheap. By the time you figure travel, set up and tear down, and other associated things, it is going to take you three hours from roll out to roll in to wash that truck. If the guy is a jerk, and most truck drivers are, (Having driven a truck myself) it could be even longer. However, we all have to learn, so at least $100.
But I am not hungry.