not that surprising...once you have enough money to cover the basic necessities the rate of return on happiness goes down big time.
I mean if you think about it, with just 60K/yr you can afford 99% of the stuff a millionaire can(provided you don't live in California). Sure you aren't driving a Ferarri, and don't live in a bigger house, and don't go to the same restaraunts or take the same vacations, but chances are you have very similar electronics(maybe you keep yours longer), eat the same food(or at least can afford to) and sit in the same train/traffic on your way to work and shop at the same grocery store.
And if you think about it, most millionaires don't drive Ferraris, they drive average cars like Accords and Camrys and F150 trucks, so chances are, the guy making 60K/yr probably own a better car than that millionaire, because he wants to appear to others as if he has money.
I'd probably extend that to about 100K, but after that chances are you have way more responsibility and won't even be able to do anything with all that money.
Reminds me of this anecdote, can't say word for word, but it was something to do with a rich guy going to mexico on vacation, meeting a fisherman and telling him that he should work harder. The fisherman asks him why do you do it, and he tells him so that he can retire at 65, buy a house at a little village, so he can fish and spend time with his family. And the mexican says but I have that now, and I work only a few hours per week doing what I love.