The cheapest way to get from SeaTac Airport to the pier is to take the Seattle light rail from the airport to the main station in downtown Seattle. At that point you are about 4 blocks from Pikes Place Market. Cost is about $3pp one way. Then take a cab from there to the pier at a cost of under $10 per ride.
Your other option is to get the transfers from the cruise line to take you from the airport to the pier at a cost of about $25pp. A cab from the airport will run you between $50 and $60 per ride. Another option is Uber or Lyft and cost will probably be $30 to $40 per ride.
Hotels in Seattle are expensive so to go in a couple of days before or after in order to fly nonrev might not be smart. If you book your cruise far enough in advance you may be able to find cheap tickets to Seattle.
If you are staying out by SEATAC Airport, the Mango Thai Restaurant and Sharp’s Roaster Ale House, offer you a 10% discount on food. Both located across the street from the DoubleTree Airport at the airport.
Discounts on hotels is difficult to find in the summer in Seattle. You want to book early and expect to pay a bit more than you would expect to pay in other cities around the USA. The cruise traffic in Seattle is huge and keeps the hotels full around the cruise departure days during the week which is pretty much every day.