United Airlines has Seattle to Singapore for $470 round trip for departures from late August into March, except December holidays.
By setting a multi-city itinerary on Google Flights for Seattle to Honolulu, then Singapore on the following day, you can pull up a 23.5 hour transit overnight in Honolulu on the outbound flight.
United Airlines Seattle SEA to Singapore SIN
Add Honolulu 23.5 hours transit layover outbound.
United Airlines Seattle SEA to Singapore SIN via long overnight Honolulu layover arriving at 1:27pm and departing at 1:00pm following day.
United Airlines Seattle – SFO – Honolulu (23.5 hours) – Tokyo NRT – Singapore – SFO – SEA
Loyalty Traveler – United LAX to Honolulu, Guam, Hong Kong, Singapore $519-$533 (May 7, 2019).