Not the slightest idea Darren but here's what i might do if I thought it appropriate and could talk to the decision maker in a sensible way. it may also depend upon if this is going to be an ongoing contract.
So, offer to do one bus and charge what you think is a realistic price. Once completed you'll have a better idea of how long it is going to take and how good a job you can achieve. You will then be better placed to work out the true time/cost/profit that will go into the project.
Then negotiate the price for the rest of the fleet. One advantage of doing one bus only is that if you do a good job not only has the fleet operator not spent a lot of money and got a bad job but he will also be confident in the quality of your work and more likely to pay your asking price.
It may also mean you void an expensive mistake/expensive under-charge.
hope that makes sense and that is is helpful.
Roger