From the google machine:
DEN-PHX: 13:52 driving time/862 mi. Assuming 35 mpg, that's almost $60 one way in gas alone
On Frontier (terrible, I know): 1:50 flight time, $87 round trip
That gives you 5+ hours on the ground (an hour either side for security) and over $30 in PHX to grab a Whataburger and some Hop Knots and then fly back to DEN. Time > $ to a lot of people.