Depends on the number of people, really.
Sample annual trip just made (CA to GA): 2438 miles.
Airline cost per-person, cattle-class [not adding in taxes, fees and per-bag costs or whatnot they stick you with now] started at $450 per person round-trip.
The 2005 Impala got between 29 to 30 US mpg on the highway, call it 29 to be generous (to your claim).
Similarly, gas ranged from $3.459 to $4.759 per US gallon, but generously use $5 assuming rising prices. That's $420.35 one way, $840.70 round trip.
Since this was 2 adults and one child (not young enough to fly free -- and even if it was possible, who would take a 4+ hour flight in cattle class with a squirming/upset infant on their lap... assuming they had any confidence in said infant not taking a tumble in turbulence), that $450 starts at $1350.
Car has to tack on hotels (depends on how aggressively you push it -- 2 days is possible with 5 hour sleep breaks or so), airline has to tack on transport to/from airport [either mass transit or rental car or long term parking], etc.
The big wins for me are having the trunk of the Impala available at no additional charge, no hassle with rental cars -- and most importantly, no getting handled as if I'd created a felony. The last point frankly would keep me driving even at a 2x cost factor, but you can't claim that "you're paying more for gas by driving" as that simply isn't true outside of lousy SUVs and driving solo.
And before someone brings it up -- the trains were *more expensive* than the car, and took a week to get there by routing through Chicago. Get Amtrack to get a reasonable continental train going somewhere between airline and car prices but taking 2-3 days tops and I suspect a lot more folks would choose it.