After estimating the size and running some quick numbers, I don't think you're that far off. If anything, I would guess you're a little high, but it depends on your overhead, the design/registration, materials used and how picky you (and your client) are.
I still estimate $15 a sq ft, based upon the total square feet of material I'm using, including design, and I try to avoid any seams when the vehicle and budget permits. Discounts vary, but I would entertain a 5%-10% discount on an order of this size, based upon volume alone. If there were any efficiencies or savings that could be generated through careful planning and design, these savings would be passed along too.
A signed contract with a commitment schedule to do all the vehicles would be required to process the order and receive the discount.
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