The drive is mostly lovely, but long. And if you hit Seattle during rush hour then it's painful, so do not be in a hurry! Give yourself plenty of time.
There's Amtrak; they have a bus or a train, so be sure you know what you're getting if you book. But they don't run all the time (the train, that is), so you might end up having to stay a night at a hotel. There's also the fare (shuttle, cab, etc) from the train station to the airport. Remember, Seatac is approx halfway between Seattle and Tacoma, which are around 30 miles apart.
So it starts adding up, but it's still going to come in less than $250 per person, even if you took cab to Amtrak, took Amtrak to Seattle, cab to airport-area hotel, stayed the night, then took a shuttle (most area hotels in seatac have free shuttles) to the airport the next day.