We have lots of immigrants in the past from Europe and the present from more places like Asia and Latin America that come to the US to live, for (more or less) a “better life”. What are the stats the other way? Leaving America for a “better life” somewhere else.
I don’t want to count people returning to their homeland or their families. But American citizens who just pack it up and say “You know, I bet Denmark/Costa Rica/Thailand is a better place than this”.
According to this site there were 3,784,693 Americans living abroad in 1999. It’s not a complete list (not all Americans abroad are registered as such), nor does it take into account why they’re living abroad. But it’s probably as close an answer as you’re going to get.
Part of the reason you won’t get a definitive answer is that there is no form that Americans can (or have) to fill out when they leave the U.S., giving their reasons for expatriating. It might be interesting if there was.