Pricing is all over the place thanks to lowballers and lowbidders.
Pricing is based on what you want to make mostly, not what others say or recommend.
I would see how many pieces they have to wash on that day, not how many pieces they have totally as they all will never be there on that day, don't fall for the trick of how many pieces they have, only ask how many will be washed on the day and that is what you base prices on. It does not matter if they have 1,000,000 trucks, you can only wash so many on a day and only so many will be there as some are working, in the shop, on the way back to the yard, etc.....
Depending on how fast you wash, if you have to brush them, what cleaning method you use I would wash one of them to see how long it takes you so then you can see what you want to charge based on how long it takes to wash them all and what you want to make.
Prices can be from $10 to $50 depending on many factors like mentioned above. Everyone's price will be different based on their overhead, profit margin, how fast they can work, etc.....