Has anyone ever seen/done an examination of the relationship between income and vote across all CD's? It's doesn't seem at all obvious what the graph would look like - my general understanding of the situation is that within pretty much every region of the country (except maybe the west coast), the richer areas are more R and the poorer areas more D, but that D regions overall are richer than R regions overall.
I haven't crunched the numbers, but I see little reason to believe there's any correlation between income and voting behavior. Both parties have plenty of poor and plenty of wealthy districts. Again, it's all about ethnicity.