That average payment is for all Americans, not just working-class. The bottom 20% of American households spends only $3559/year on transportation (overall, not just on car payments). So either their car payments are much lower, or they just don't own many cars, but in any case they don't have many $500 car payments to forgo.

http://uk.businessinsider.com/how-high-income-and-low-income...