Why are white and black display assemblies different prices?

For a pre-Unibody Macbook, a white display will always be less expensive. The black Macbooks (2006-2008, A1181) were the top-end model in each generation, and so were much less common than the white ones. Since most of the available parts are cannibalized from salvaged computers, the prices of used parts reflect their availability. You'll see a similar price spread in other white vs. black case components. However, aside from the sometimes faster logic boards and sometimes larger/faster hard drives in top-end models, there's no functional difference between A1181 components in a white Macbook and a black Macbook.

I think there's a practical advantage in mixing black and white case components: You make your Macbook far more identifiable in the event of theft.