Frontier has round-trip flights from Dallas (DFW) to Seattle (SEA) for $38-$47, NONSTOP. Flights August to September. Departures on Wednesday and Saturday. Origin/Destination reversible.
Frontier fares do not include a free carry-on bag. Only a small personal item, like a Jansport SuperBreak backpack is free. You can avoid paying for seat selection if you accept the randomly assigned seat prior to boarding.
Frontier seats only have 28-inches of pitch; 2-to-3 inches less space than most other airlines.