It's actually pretty well documented that if the companies failed (and both almost did, by almost I mean within weeks of failing), Elon would have been ruined. At the point where he invested in Tesla and SpaceX, he wasn't actually billionaire, and his companies certainly were not liquid. The one out Elon had was having Google buy Tesla (which almost did happen). A few sources:
"In the meantime, at SpaceX, Musk and top executives had spent most of December in a state of fear, but on Dec. 23, 2008, SpaceX received a wonderful shock. The company won a $1.6 billion contract for 12 NASA resupply flights to the space station. Then the Tesla deal ended up closing successfully, on Christmas Eve, hours before Tesla would have gone bankrupt. Musk had just a few hundred thousand dollars left and could not have made payroll the next day."
He may have been extended, but that doesn't mean he was almost in the poorhouse.
Billionaires getting ruined doesn't happen any more.