I think it has more to do with production volume than material cost in this case. Think of it this way: it probably costs about the same to design and tool up for production of a 10" LCD as it does for a 14" LCD. Since the cost of the tooling and the research and development must be factored into the price of the LCD screen, each screen must pay for its portion of the total tooling and R&D cost (based on how many the marketing guys expect to sell.) Since there is a much bigger market for 14" screens (they're big enough for your parents to read email without squinting too much) than for 10" screens, the 10" LCDs each have a much bigger share of the total cost to pay- at least our parents all want to run PCs in their cars.
The solution- Get more people to install LCD monitors in their dashes.