If you look at the Bureau of Labor and Statistics they will happily provide you with more detail, for instance the median salary in a field for the top 10% of employees versus the top 90%.

I don't know how you'd keep that updated though, you can hardly say "the amount the richest 10% spend is the rich CPI" because it's a silly statistic. I don't know how you can reasonably say "the minimum amount required to be rich", it just seems silly on its face.

No, I don't think there's much of a way to make it work. It's probably more directly useful to understand whether people are saving money or how much debt they have, though that's going to lag.

I mean, or just ask people. The government is fairly good at doing that.