how else can i "estimate" a job if i dont know how much material i'll be using.
Why don't you know how much material youwill be using? If you know the car make/model/year then you can get the sqft needed from your design software (like PVO)
From that base estimate you can quickly come up with a price for a given area. You can generalize by type... Mini's/Compacts, Small, Midsize, Small Truck, Full Size Truck, Small SUV, SUV, Box Truck, Trailer.
Explain that there are four elements that define the costs for a vehicle advertising program... Design, Print Production, Vehicle Prep/Installation, and.... BUDGET!
Based upon your interview about their needs/goals ("profiling") you can explain how you can tailor an advertising program that gets the job done AND meets their budgetary requirements.
Maybe you mix print elements with cut vinyl... maybe it is a half wrap with door decals. Whatever. Then you can also "coordinate" the marketing by creating business cards, point of sale posters, rack cards, sales sheets, window decals, banners, bandit
signs, etc.. to MULTIPLY the effectiveness of the vehicle design.
Think about Geek Squad. I will bet you visualized that black & white VW with the orange emblem... They standardized on a vehicle design and it became the core of their brand identity and marketing campaign.
What if they just said... how much to wrap my VW?
Sell the SOLUTION...
The guy wants to increase sales & visiblilty. Show him how you can do that for him. A vehicle program is just one piece of the puzzle.