2008 worldcon site selection|
All worldcons give away their excess cash, that is how Torcon got some of its surplus, from previous worldcons.
Chicon V gave everyone a $10 rebate.
However you are right, more people will attend if they can drive as opposed to flying. A car is a sunk cost while a plane ticket is an extra expense.
Midwest fans used to drive to west coast worldcons, although there were fewer cons in general back then.