I think if happiness was a function of money, then it would very much look like the sigmoid / logistic function. You have a linear region where there's a very dramatic relationship between money and happiness (a $100k salary makes life so much easier than a $30k salary), but there's very small difference at the different ends...a $5k salary will not improve your life much over a $500 salary, even though there's an order of a magnitude in difference - just in the same way that $50MM won't make you much happier than $500MM.

And this is something I feel is important, because people tend to compare extremes. It's always poor vs rich