Public Transit Saves Money
report from the American Public Transportation Association shows how much money the "average" commuter would save in each of 20 different US cities. The US average is $825 per month. Figures are based on gas prices of last Friday and reflect an average monthly parking cost of $161.56.
Seattle is number 5 on the list with a monthly savings of $973. Using a rather simplified calculation that ignores such things as licensing and taxes, you can calculate your own savings here.
Given the trend of oil and gas prices, savings are likely going to increase, even if public transit fares also keep going up.