If the dealership has used decals on their vehicles before, then they already have an idea of the price they're willing to pay and are probably looking for a deal. You can always ask them what they've paid in the past and then see if you can, or even want to, beat that price.
i have customers tell me they'd like me to do their decals because i'm reliable and fast. if they have order from someone else before, i will ask them what they've paid in the past. if it's a ridiculously outdated low number, i will them that it is and what an average shop will charge them now days. they usually complain about having communication issues with other companies, or missed deadlines. if i am willing to beat the price, i'll give them a price. if i'm not, then i point them in the other direction. if they're a difficult customer, then i don't bother quoting at all.