The Dow is dollar-weighted (not saying you didn't know this staffpro, just thought I'd mention it for newbies). So, it's easy to figure out.
The Dow divisor right now is 0.122834016, and GM's price is 4.36, so if GM went to 0, it would decrease the Dow by 4.36/0.122834016 = 35.5 points...