United Airline has round trip non-stop services between Los Angeles(LAX) and Denver (DEN) starting from 53$ in selected days. Fares around 70-80$ are common too.
Airline: United
Class: Basic Economy, play note the many restrictions United Airline’s basic economy class here:
Travel Date: Selected date in November, December and January (See chart below)
Book your trip on Priceline, Travelocity, Cheaptickets, Cheap-o-air