How much to have a 52 foot trailer and tractor (big rig)washed

I worked at a recycling center a few years back and the boss tried his hand at truck washing by installing a power washer and creating a tall walkway for the techs to reach the top. He charged $50 for a full brush and soap wash (including the top of the trailer) and the drivers said it was a good price, they usually paid $75 to $100 at other places. For a brush of the tractor only and quick power blow on the trailer he charged $35.