Funny how much a billion is today.
Tesla Motors for example is "only" worth 700 million, and most of this is still from the initial investments and loans (Alone 465 million USD from the US DOE as interest-bearing loan), not by selling cars and car components. It actually still has a negative cash flow and has yet to turn a profit.
Same for SpaceX - no hard numbers known, it is estimated at 1.3 billion. 875 million are estimated to be Musks share of it, he invested $100 million at the start into it, then it started gathering loans from NASA in the context of the COTS programs. SpaceX is still in the same early phase of its existence as Tesla Motors: Negative cashflow, many loans and money from investors who wait for it to turn a real profit one day.
Musk had only one really big company, and that was Paypal. Which he sold for 1.5 billion at the height of the .com bubble, which was the really best that he ever did - he left it when it became a real multi-billion company.
PS: Musk himself says that $1100/kg is very possible. Which isn't far away from what I said before in the thread, and still a major achievement. $100 is just fantasy even for Musk, when he has to estimate its feasibility.