Yeah, the market cap of SpaceX is undefined.

And since Musk owns 2/3 of the company, and seems to be a man with a mission, it's not at all clear that he'd sell to anyone.

Then again.... SpaceX was apparently built on around one billion dollars. So for $10B (Charlie's suggested figure), you have a fair shot at building a SpaceX from scratch. Start by offering to triple the salary of everyone at SpaceX if they come work for you.

I doubt many folks work at SpaceX for the money, so even if it was 10x I doubt many would leave. You don't build a SpaceX from scratch unless you have someone like Musk driving it.