Got a cite for the $500m figure?
I think SETI@home ran on $5 mil, and the Allen Telescope Array will cost 3 times as much. Throw in several million for other projects and you've got $25 million for maybe 15-20 years of effort (early 90's to early 2010's).
I support SETI at that kind of financial burn rate, although I wouldn't support it at the much higher rate you're talking about.
