I pledge my support for Mr Trowel: half a million! Still, I'm surprised that out of three responses, two are incorrect, so to clear things up...
- half a billion/a half-billion (the second example is an adjective) is 500,000,000
- "thousand" doesn't receive an "s". You're not counting "thousand"; you're counting "five hundred thousand ______" Five hundred thousand dollars, five hundred thousand widgets, five hundred thousand shoes, etc...