The Spokesman Review analyzed the cost of travel from Spokane to Seattle using three different modes of travel (four if you consider the difference between a hybrid and a gas fueled automobile). They looked at planes, trains and automobiles. The published results are in a table format here.
I decided to calculated what it would cost to drive my Blazer to Seattle using Google Map's estimate of a 608-mile round trip from Post Falls, and I added 20 miles for cruising around downtown Seattle for a total of 628 miles. I came up with a trip cost of $105.27. Because I live in Post Falls, I also used $3.19 a gallon instead of the $3.65 the SR used.
Using the SR's gas price, my trip would cost $120.45, but the real cost is going to be slightly higher because I would have to gas up in Seattle. So if it were just me traveling, the cost of train and air travel would fairly competitive -- especially when time is a consideration.
How much would it cost to drive your car to Seattle? For those who are story-problem challenged, just divide 628 (for Post Falls residents) or 644 miles (For Cd'A residents) by the miles per gallon your car consumes and then multiply that answer by the price per gallon you pay. Have fun!