Not for nothing, but given the quality of the product and how "niche" it is (what else would you see this design on?), $400 shipped to cover the roof sounds inexpensive, at least to me. Don't apologize guys, this sounds very cool.
I am going to ask a local guy near me who can print vinyl and see if his price is similar to yours. I suspect the DeAutokey product may be a better quality, so if the prices are similar I would definitely be in on a group buy for the roof size sheet.