Frontier is having a three-day sale for purchase by July 12 and traveling August 21 to November 7. All these flights are on nonstop routes to/from Denver. These are not knock your socks off fares, generally about $20-$40 less than what others are charging, but still, they're lower and nonstops whereas other airlines fly many of these routes with connections.
For example, Denver to San Diego is $119 each way, and we've seen that route much lower in the past. But check the sale out if you wish.