haven't looked at the cards but it probably has to do with what type of cargo is needed...
I haven't played the game but if I recall correctly from looking at the cards some Cars are more expensive to put out than others... I'm just guessing but it would make some sense...
Yes, Coal is the least valuable, Oil is worth more, and boxes are worth the most.
As a result, Hoppers are cheapest and have the most efficient VP/cost ratio, while Boxcars are most expensive (and lease efficient VP/cost).