The only way from Aurora to Boulder is by public transit. You should expect to be able to find tickets starting around $13, if you search flights from Colfax & Billings to Boulder Transit Center. Take public transit from Aurora to Boulder, it's your best chance!
The trip to Boulder is an excellent trip for the day or a nice trip for the weekend from Aurora. The cheapest way to get from Aurora to Boulder is $13 and the quickest way takes just 1 hour 47 minutes with the public transit. The earlier you can book your trip to Boulder, the better. Public transportation prices do not fluctuate like flights. Instead, they're either flat rate or priced by distance traveled. If possible, you can book your tickets via an app or alternatively at the station upon arrival.