Very interesting.
I grew up Baptist and we were poor, as was most of the congregation in the churches we attended. The nice thing about that was that there was a real sense of community and sharing. Clothes and household items were regularly passed around and pot-luck suppers happened almost every week.
As a teenager, my parents switched to a non-denominational church. The congregants there(as well as my family at that point)were pretty solidly in lower-middle and middle class.
I've been a practicing Quaker since my late teens. From articles I've read and my own impressions, Quakers seem to be more educated and middle to upper-middle class in terms of income.
I think a lot of it comes down to the kinds of personalities that are attracted to certain careers also finding a home feeling in a faith group. Baptists seem to have a lot of factory workers and tradesmen. Non-denominational groups seem to have a lot of small business and shop owners, farmers, and musicians. Quakers have tons of teachers.
I've met very few people in my Christian life that 100% agree with what their denomination teaches/practices. Generally, affiliation comes down to the overall style of the denomination and not the finer points. So income correlation is most likely about style preference, not beliefs.