Obviously it doesn't buy happiness at simplistic level, as in their are unhappy, I'll rich people and happy, healthy fairly poor people.
There is a correlation between money and health at a population level though.
Were not poor - we're above average earners but live in a rural area where most people are inherited property rich and live in gigantic two family houses with the parents' parents who provide childcare (though I wouldn't want to live in this set up it's a money saver!) or have had a house built for them on family land a dozen meters from one set of grandparents, who provide childcare... (abroad) and very few young families are totally self financing with rent or mortgage to pay for - which means that our disposable income has been tight at times because we are "incomers" to the area with no local family or inheritance.
My mum, who lives 800 miles away and has a pension larger than my salary and has long since paid off her mortgage, absolutely lives the phrase
"It's only money"
I was once stressed about having scraped the side of my car on our own gate while parking, which was going to cost a month's salary to fix - entirely my fault obviously, but my mother interrupted me telling her to say "ah well, never mind, it's only money, nobody's hurt that's the main thing. Your dad and I are quite stressed too because the gardener doesn't understand my vision for the garden and is threatening to quit" 
My parents were under similar financial pressures to us 30 years ago, but now they are very comfortably off indeed they seem to have undergone memory wipes and live the phrase "it's only money". It is "only" money but sometimes it's a big stress factor.
Obviously for some people it's a thousand times worse. Plenty of people have killed themselves over money or died for lack of it - unable to afford heat and food or in other countries healthcare.
So sometimes it isn't "only" money.
Assuming money isn't important is often acutely insensitive and a bit smug IMO. It's an assumption that only people who are comfortably off can make.