Now, most certainly the scale changes. That is, if you compare me to when I'm worth $1000 dollars to when I'm worth a million dollars, that's a 1000 times difference. But it's not the same 1000 times difference if I were to go to a billion dollars. The first 1000 times is worth a lot more. The difference between your first billion and your tenth billion has got to be vanishingly little. But that first 1000, from a $1000 to a million? Hoo hoo hoo!
I think anyone who says money doesn't make you happier should go a day with miss-meal cramps.
For my own part, I've been a grad student with a $20k stipend for years, and am pretty happy about it; I even feel reasonably well off, since there are tons of ways I could be more frugal if I really needed to (I spend a lot on beer and eating out). I guess having $30k or $40k or $100k might be nicer, but I can't imagine it'll be that big a difference? We'll see, I suppose.
The real problem is that people don't know what'll make them happy. Things that money buy don't make you happy? That's bullshit. Yes, there are very important things in life that money can't buy, but there are quite a few things that money buys that does get you happiness. For me, money has bought me independence of action that has made me very, very happy.
So people don't know what will make them happy, and the researchers are asking the wrong question and reaching the wrong conclusion. What keeps people from getting happy is that they think an extra 400 square feet of home, when they've already got 2500 square feet will make them happy; or that a car with 300 hp will make them twice as happy as a car with 150 hp. And then the researchers are coming along and saying hey look, these people have more money but they ain't happy, so money doesn't make you happy. No, all it says is that buying the wrong things with money is not going to make you happier.
Don't tell me that those billionaires that spent those millions of dollars with the Russians to go out into space weren't made deliriously happy with what their money bought them. They just knew what to buy to make them happy.
Admittedly, I expect it'd take more than $20k/year to live comfortably if I had a family!
Independence to take action means also being able to afford to take that action; not just that you can learn to avoid expenses.
I know plenty of people doing startups, and I could even throw $10k their way if it'd help out (I've offered), but they just don't need it. The web side of things is so capital-free these days that they have no trouble self-funding, and what money they do need is easy to get from a few consulting gigs here or there (web-dev stuff seems to pay insane hourly rates).
I can imagine things I would do if I had infinite money, but they're more pie-in-the-sky "establish a fund for [x]" sort of philanthropy things. In my normal life, I just don't run across action I really want to take that is money-constrained.
In a similar vein, another recommendation is to favor differential experiences over static improvements, e.g. traveling rather than buying a nicer car. Your expectation of enjoyment from the car is overestimated because you consider the enjoyment relative to your current state, discounting your own adjustment once you have it, while travel is continually changing by definition, both with your departure and your return.
Interesting subject, hacking your own enjoyment of life.
The Wikipedia article (and even the choice of the word "treadmill") suggests that people maintain a roughly constant state of happiness despite improvements in their life. But it also works the other way -- people return to a baseline of happiness even with huge disasters in their life.