One of the low fares today from United Airlines is a $109 round trip for Los Angeles to Denver or reverse. This looks like a direct attack at Spirit Airlines low fares for the route.
The $109 fare is available for dates between September and the first week of November. October has the most dates with this low fare.
United Airlines Los Angeles LAX – Denver DEN
$108.20 Tuesday October 20 – Tuesday October 27
Filter Google Flights by airline for United to find the $109 departure dates without Spirit Airlines fares clouding up the search.