I think by "wealth" Kitsune is referring to a model's apparent overall socioeconomic status, not just her income. (Although you raise a good point that the two variables in her question are not perfectly independent.)
Personally I think the real challenge in testing this would be separating out the impact of a model's individual attractiveness, since we know
there is a correlation between wealth and beauty, and I would assume beauty is itself a driver of tip spend. I'm sure you'd find a shortage of both beautiful girls in favelas and ugly girls in mansions to use as test subjects.
But then I'm guessing Kitsune has a much more specific question in mind, like, "Should I keep this expensive artwork in my bedroom or not?" Something like
that would be much easier to test for. (
@Kitsune?)