1) If they win the self driving race that will be incredible value beyond just manufacturing cars.
2) Battery manufacturing in general is huge on its own.
3) The roof top combined with domestic battery side is potentially significant in replacing (full or part?) domestic power companies
4) Insurance - Musk said this could grow into 30-40% of their business
5) Super charger network gives them a first advantage to replace petrol stations to a degree.
6) Elon factor of what's next.
And dont get me wrong, I do feel its gone beyond reasonable value, but there is more than a car company there.