There's also the economy of scale thing. For example, one reason Dell can afford to sell 2005FPWs for under $500 on a regular basis is because they are wickedly popular, which allows them to manufacture the large displays in big enough production batches to realize major cost savings. The 7" ones are in far less demand, and consequently it is more expensive to produce small batches of the panels.
Also, car audio and video devices can sometimes be stupidly expensive, just for the simple reason that their target demographic (middle class teenagers living at home)generally has a lot of disposible income, so that could play into the weird pricing as well.