The Car Dealers should be making enough money to pay you pretty well.
If it were me, I would come up with a pricing structure and charge them a flat rate per pound of whatever you are making. That way you can't lose money unless you do your math wrong. If whatever they order ends up being gone before the end of the event...that's too bad. Just my opinion.
It has always been my theory that if you are not making money its known as a hobby. My hobby is for me...business is business.