Southwest has round-trip flights from Seattle or Portland (SEA, PDX) to Denver (DEN) for $119 NONSTOP. $60 each-way. If you’re traveling with multiple people, you can save more by buying the $500 Sam’s Club gift-card which is on sale for $450. Flights August to November. Origin/Destination reversible.
A Carry-on bag, two checked bags, and also a personal item is allowed for free on Southwest.
Click for more details and booking links Nonstop Flights: Seattle or Portland to/from Denver $119 r/t [Aug-Nov] – Southwest