The problem is this.... what is a good price for you doing the work??
If you go by some of the advice here, you will price yourself out of the job...
So do you actually want the job?
If you do, work out how long each seat will take, and how long you take to get the water and electric etc each time, then compare that to what you charge normally.
I would personally charge the price Derek has suggested, as it is a real balls ache doing coach seats, so I woul dtry to price myself out of the the job but if I was to get the job it would really have to be worth it.....