I'd say more than $35, not that I've ever taken a cab from Seattle to SeaTac, but I've picked people up from the airport when I lived there and it's not a short distance.
You don't have to go through belltown to get to the light rail. Victoria clipper comes in on pier 69 and the nearest place to get on light rail is third and university. It's like 4 or 5 blocks.
Ok, you don't HAVE to go through Belltown, but there's no way that it's just 4-5 blocks, it's way more than that (especially to 3rd and University, that's further away than Olive and 4th which is where I had her) And it WILL take you 20mins + with luggage. With Seattle hills. No way.