GM's value is zero. The stock price represents a nervous dance between shorts and longs who try to force sqeezes. (And I suppose there are gamblers who are betting that the government will bail them out without a ch 11 style reorg.)
I love Tesla, but if I had to place my bets, I'd think it more likely that you'll be able to buy a Corvette in 10 years than a Tesla roadster.
The most likely discontinuity ahead for GM is a zeroing-out of all current stockholders. For Tesla, there are certain discontinuities -- such as acquisition for technology -- that pay off stockholders even if the nameplate disappears.
There is the Net Present Value (NPV) of GM, which is the value of it's future revenue streams - future losses. (Future revenue streams include some bailout money, etc.) Currently, the expected NPV of GM is negative.
Then there is the share price of GM. This is equal to max(NPV,0) because even if the NPV of GM is negative, you won't lose more than the value of your shares. So a share of GM stock is actually an option on the NPV of the company. This is nonzero, since there is a chance GM will recover.
Fun fact: to increase the value of GM's shares, you can either increase NPV or increase volatility.
[edit: my use of the term enterprise value was wrong. I think I meant NPV, but I'm not 100% sure of terminology. Thanks sachinag.]
But, woooo NYU.