Like the guys said, there are just too many variables out there:
What you want to make per truck or per hour
What your cost is for fuel, soap, brightener, degreaser
What your labor cost is
What your business insurance cost
What your vehicle expenses are
Etc..........
What some guys making down south are probably not what the guys up north are charging as they don't work the whole year so they might charge more during the summer to save some for the winter.
I would say to not do anything under $12 and with certain fleets, give discounts based on higher numbers and only on what you will be washing.
A lot of fleet owners/managers will tell you that they have 60 pieces but when you get there to wash, there might only be 30 or 20 or 50 so you just never know.
You need to ask how many will be washed on average.
I know a couple guys that started implementing an 80% charge of what they told you they would be washing so that when you show up, you will make at least 80% of what they told you that you would be washing so that you are making money, especially if you are there from out of town. If all the pieces are there then you are making more money but if they are not all there, you are still making 80% at least so you are still making money.