I don't know if you've read much Robert Sapolsky, the well-known researcher on stress, but one of his more salient points is how economic inequality really is bad for everyone, in terms of quality of life as measured by stress levels and such. In the US, the struggle of the poor is obvious. But the rich almost have to isolate themselves from both the poor and the average person, which is very bad for both your stress levels and a general sense of trust in society, which affects everyone.
The rich tend to live longer, though.
As for money and quality of life, it goes something like this: starting from zero, the more money you get, the more your quality of life improves until you reach a point where you have a home with no debt and you can manage your ordinary bills and living expenses without any trouble (and pay for your children's education, if you live in a country where that is necessary). After that, extra money has almost no effect at all on your quality of life.
(I think it was Sapolsky who also pointed out how average income is funny in countries like Mexico and Russia because although it obviously and mathematically exists and has to exist, no one actually earns that kind of money in these countries. Most people are much, much poorer, and a select few are ridiculously rich.)