You could use $3 - $5 sq. ft. for estimating, but you have to figure in everything to give an accurate quote.
A better way would be to figure out how many hours it will likely take, total up your burdened labor rate, and mark everything up to cover your operating cost, overhead, and profits. Like a previous poster pointed out, 500 sq. ft. on a box truck with no rivets will take far less time than wrapping a VW bug.
The best way is to just charge per hour until the job is done. I'm 62 and have been doing this all my life and this has never happened to me, but I have heard stories...