Just trying to do the maths (in the aussie dollar)
FH3 Standard edition: $100. 350 cars. That's $0.29 per car. Except it's actually much lower because car licenses are only a %age of the development budget.
FH3 Deluxe edition: additional $20. Additional 15 cars. That's $1.33 per car. Other items have a development cost (i.e. new events) but are relatively negligible.
FH3 Ultimate edition: another additional $20. Additional 42 cars. That's $0.48 per car.
There is no single fixed cost per car - some licenses are going to cost more than others (i.e. Ferrari + Lamborghini). So I guess the most expensive cars would be shoehorned into the Deluxe edition car packs.
The only other factor is that a vehicle license cost might not depend on sales numbers. So if they're expecting only 50% of people who buy the game to buy the DLC, then they need to double the price of cars in DLC to make the same margin.