This one http://www.yqcomputer.com/
says $4,000,000,000 (4 billion) back in '98. I'm sure you could add another
$1 billion since then.
I can't speak for them.
It's good to be da freeloader, eh?
Imagine how much better Linux would be if some of these developers were paid
to write apps. Maybe the Gambas project would bear fruit, instead of
languishing because the creator has only so much time.