It *is* well known that religiosity declines with increasing income and education, and I *am* surprised that you didn't know that.
It is well known, yes, but is it, in fact, true? I also tried googling, and found little evidence either way. There is much on correlations between national GDP and religiosity, but very little on whether this holds true at an individual level.
I only found one study which compared the religious affiliation of individuals with their income, for which I can only find second hand reports (like
this from NYT, not the original data. This one doesn't give religion vs. non-religion, but breaks it down into religious affiliation, one of which is secular. It also only reports the percentage of households with income over $75,000.
Now, in this study there are slightly more rich secular households than the national average, but only slightly more, so the correlation must be weak. A far higher percentage of Jews, Hindus and Episcopalians are wealthy.
So, commonly known fact, but if you want to claim as actually being well-supported rather than one of the many things that people know without evidence, you'll need to show us something.