Where did you get the idea that a SpaceX launch costs $500 million? The entire Commercial Cargo development contract with SpaceX was $400 million. The subsequent commerical resupply service contract is for 12 missions for $1.6 billion. If you spread the development cost over all those flights, it comes out to be ~$150 million per launch. (Source: http://en.wikipedia.org/wiki/SpaceX#Commercial_cargo_contrac...)
I think you misread the previous comment. It was referring to $500 million for a contract that included many launches. I agree that it could have been more clearly stated.
